(updated Jan 6 2017)
Here's some detailed advice on how to pack, gathered with personal obervations from someone who's been traveling constantly for 40 years. Since this is more of my own style, this post belongs here rather than in Travel Academy.
I separate my luggage into 3 things.
A purse/fanny pack/manpurse, whatever you want to call something small, usually a sling bag of some kind, that you can shift around so the strap is tight around you and the bag opens in front of you, around your chest. This is where the bulk of your money and your passport goes. Separate your cash money into at least 2 piles. One goes in your wallet or if you're like me, a neatly folded wad of cash in your pants front pocket, in a money clip if you have it. The other, which consists only of 10,000yen bills, goes into the more secure purse like a bank to withdraw from when you run out of smaller notes. Credit cards also goes in your wallet separate from your passport. If it has a separate compartment, your purse is also a good spot to put receipts, the ones you may need to show customs when you return to your own country. And while Japan is a relatively safe country, it's also a country with a lot of sensory distractions, so in case of accidents, it maybe a good idea to use a purse/bag(man-purse) that has a zipper and not an open bag.
A good size backpack or messenger bag, this is where you put your day-to-day optional gear. water bottles, hand sanitizers, camera gear, sweat towel, anything you might need but you can spend the time dropping the bag to pull out. something with a comfortable strap, and enough space to hold some impulse purchases. Mine is usually bogged down with DSLR camera gear and multiple lenses, accessories, ... but it still has space for a nalgene bottle, smaller towels, an overnight change of clothes(t- shirt, underwear, socks), just in case something gets dirty or wet, I'm not one to care much about umbrellas when it rains or getting my shoes wet walking along the beach. A reminder that this bag/backpack should fit international carry-on luggage size.
A Pacsafe sling bag like this, is around the perfect size. I use a similar sized fanny pack. This should be comfortably tight right up against your chest, with whatever protection you think you'll need, RFID blocking lining, anti-slash lining. This is where your most important things go, and it maybe worthwhile to find one small enough to fit inside a small hotel room safe. Mine holds, my wallet, passport, international driving permit, the larger stack of cash, usually 1 or 2 small pocket cameras(last trip was Canon G12 and Ricoh Theta S + selfie stick), UV-protected prescription sunglasses in it's own bag, earplug headphones, a pack of tissues, a pack of wet wipes, and a small wallet for camera memory cards(which can also hold my own SIM card if i bought a data one at the airport).
Some people like using small 2-wheeled rolling business suitcases, and for the most part, it's fine in the city if you're just going from station to hotel, but it gets tedious to have only one hand available when you're walking everywhere with it in tow, not to mention in crowded areas, that little rolling case will trip other pedestrians, just as low umbrellas will poke people in the eye. While it has it's place for the countryside or if you're a light packer going on a business trip, I cannot recommend it for city shopping use as it serves no purpose and it's behind you where you can't see if it's in other people's way, unlike baby strollers(for babies, not for dogs) which are in front of you so you can direct people out of the way.
As for the checked luggage...
While I've had more than my share of Samsonite, Halliburton, Victorinox, Delsey, Tumi, Pelican, and other named brand suitcases both hard and soft varieties, for the last decade or so I've mostly traveled with some cheap(~$60-80USD) no-brand giant soft trolley with semi-rigid sides. after realizing airport baggage crew will rough up your checked bags in anyway they can, i might as well not pay $339 for some TravelPro that’s barely any better than the cheap crap i get that lasts just as long. (shortest lasted 1 trip, got one of the wheels broken off and the front pocket zipper ripped apart. longest lasted 10 years: 2003-2013, same cheap brandless suitcase). Additional security measures depends on what you're bringing. Remember airport thieves goes for convenience and they can open zippers from the other end. Lock up your suitcase, use a cover bag and lock over that with a combination lock luggage strap. Colourful cover bags also make your suitcase stand out for easier recognition on the conveyor belt.
If luggage is expensive in your country, you may want to consider buying one or more in Japan. Surprisingly, you'll find a great selection at Yodobashi Camera(most of the larger branches has a luggage department). They sell some brand names like Samsonite, Delsey, and Rimowa, but they usually also have a great selection of no-brand big 4-wheel trolleys. For camera enthusiasts and anyone who wants bags with better padding(great for say, glassware from Otaru), Yodobashi also have great selections of camera bags(slings, messengers, backpacks, you name it...). The best and biggest branches for all this are probably the huge one at Akihabara for Tokyo and Umeda for Osaka.
On how to optimize space, it’s knowing how to pack that matters. Learn to roll your clothing. Fold in the sides then roll from the bottom up to the collar for shirts and t-shirts. It will keep the main parts of your chest and back taut and free from creases, better than a hanger bag. The folded sides could easily be hidden under any kind of coat or vest. For t-shirts, rolled up t-shirts makes it easy to identify which shirt it is by the pattern on the back, and because it's slotted into your suitcase vertically, it's also easily recognizable by colour, even minute colour differences if most of your t-shirts are black or white, and it can easily be pulled out without messing up other t-shirts, unlike a normal folded stack. For easy check-in and check-out, I never bring out my clothes from the suitcase, for one, I don't need to thanks to the rolling system, everything is in place and easy to pull out without messing everything else up, and also, while I expect hotels to put on clean linens on the bed, I can't say the same for closets and hangers, so I'd much rather have my clothes in my suitcase that I keep clean. (and I do wipe down my luggage after a trip or two, with some diluted Dettol, a liquid antiseptic, and a washcloth. as i do the inside of my shoes in case of bedbugs or other critters that could hitchhike off hotel room carpets) When packing up to go home, these rolls of clothing also function as additional shock absorbers for fragile items. Dirty clothing are rolled then placed in plastic hotel laundry bags, leftover convenience store bags, to separate them from clean clothes, or when I'm traveling with 2 suitcase, clean in 1 suitcase, dirty in the other(still in bags for easy identification if I come across a coin laundry at the next hotel).
For an additional partitioned space in your big suitcase, use a small padded camera shoulder bag or messenger bag to carry all the cables and power adapters so they don’t roll around if the suitcase isn’t completely full, and the padding prevents cables from being crimped. bring extra empty padded bags and bags of cotton balls or pocket packs of tissues that double as liner if you intend to buy fragile gifts. soft trolleys have some rigidity but also have give, so they’re not crushed flat in backpacks or shaken in a cocktail shaker of a hard suitcase.
The tighter the rolled clothing, the less likely it'll have creases when you unroll it for wearing.
If you're not also handling children, and your airlines allow 2 suitcases, bring 2. Even if you don't end up needing both, go to a Don Quixote or a 100yen shop(eg. Daiso, Lawson100, Seria, ...) and fill both up with whatever candy, medicine, tissue packets, toilet paper, cup noodles. These fillers makes quick and easy casual gifts, and they prevent the other contents in your luggage from shifting around too much or ceramicware/glassware from shattering. Bring a beach towel to cover everything before buckling the inside straps. Beach towels because they're usually thinner than your normal bath towels and they double as a poncho, picnic blanket, extra headwrap in a blizzard, wiping dry camera equipment in a bind, and they dry quickly because they're thin. Why candies and instant noodles? It makes it easier to get through those few weeks of post-travel blues and since there's so much variety in Japan and they're cheap, it's less strain on your dutiable budget and they can handle some rough handling of your baggage by the airport ground crew.
This in the picture is an American Tourister, but if you can find a no-brand for cheaper, get that instead. While the name brand may have better QA, it's unlikely they'll last any longer against rough handling by the baggage handlers. Although if you can get brand names on sale, that's always good too.
General tips on what clothing to bring...
Everyone's ability to tolerate heat or cold is different so here's some general tips on summer and winter wear, touching lightly on climate in the popular areas.
Summers in major cities(except for Sapporo) can be unbearably hot and humid. Not as humid as parts of Southeast Asia, but especially with Kyoto, it can be hotter. Kyoto is an inland basin with shallow waterways and several times per year can get over 40C(104F), and clothes get sweaty and wet very quickly. Wear something loose fitting, made with airy materials, or something that can soak up a lot of sweat. Always have some kind of towel on you, especially if you move in and out of air conditioning a lot, the constant transition makes it easy to catch a cold, and for people holding cameras or selfie sticks in their hands most of the time, it might not be the worst idea to have a tennis wristband to prevent salty sweat from crusting up your camera. Those sweaty towels, can be rinsed off at washrooms at convenience stores, but remember to ask the staff f you want to use their facilities. Good things about Japan in general, their tap water is almost always very cold. As for the other major cities, the urban heat sink doesn't make it feel any better, and while major shopping centers may have decent air con, you can be sure that there's only weak air conditioning on trains and in many buildings, especially in Akihabara or Shibuya's upstairs shops in older buildings(Dotombori/Nihombashi in Osaka, Sannomiya in Kobe, ...). With the exception that's Sapporo which averages around 15C(59F) to 20C(68F) in July, with a few days a year up to 30C(86F). The mountainous interior in general is about 18-26C, still warm, but a good deal cooler than the Pacific side, which is why higher elevation places like Karuizawa(base elevation around 950m) are popular for Tokyoites(Gero for Nagoyans, Arima for Kobeites, ...).
Winter in major cities with the exception of Naha, can be very chilly, even along the Pacific side(Tokyo, Osaka, Nagoya) even if there's not much chance of snow, a bit of humidity makes the winds feel even colder. Mountains, Sea of Japan side, and the north will be very cold and dry with a lot of snowfall as their climates are affected by northwesterly Siberian winds. Regardless of region, a good waterproof jacket is very important, the thickness depends on which of the above 3 areas. Naha, expect around 15-20C, cool, but not bitter cold. Fukuoka and the Pacific side, maybe around 5-12C with a few days around -2 to 2C and snowing, Tottori to Niigata around -2 to 5C(28 to 41F), with maybe a few days up to 10C. Tohoku and Sapporo, -10 to 0C(14 to 32F). This is not accounting for windchill. While this may seem very cold, Japan in general overcranks their winter heaters. Expect most places to have their heaters up to 25C or above, some around 28C, even on trains, so wear easily removable clothing, or have your interior layer being able to soak sweat because if you sweat out on a train and you're getting out into freezing cold temperatures, that's eaily going to give you sinus trouble and migraines. oh, and sweaty feet because a lot of the heating on trains are blown from beneath the seats. If you are from tropical areas and aren't used to the cold air, it might be somewhat painful to breathe in the air. Get yourself a face mask, either the disposable surgical masks which can be purchased in any convenience store, or buy a cloth one if you see it, that you can wash and hang dry at the hotel. Breathing through a mask will let your exhaling warm up what you then inhale just a tiny bit, and also trap a bit of moisture, making it easier to breath in when you're outdoors for longer periods of time, like if you're going to Sapporo in February to see the ice sculptures. For snow adventures, it is a good idea to make sure your extremities are kept warm. A toque(what we Canadians call a knit cap), good knit scarf, warm gloves(waterproof where you expect snow), and good non-restrictive socks with anti-slip water-resistant boots(for protection from slippery ice or melted snow puddles, best is natural rubber soles and a fairly high boot that is not hard to put on and remove, Baffin, Uggs, Sorel, Cougar, all good brands for winter boots, although it's not necessary to go too far and get those arctic -60C rated ones). Maybe even toe socks(common in Japan) to alleviate problems of sweaty feet or toes easily have fungal problems or corn if they're not in footwear that breathe eaily.
Sorel Caribou (for both men and women), great waterproofing and easy lacing. If laces are too much of a pain, it'll cut into your enjoyment time. Some Japanese restaurant and other kinds of indoor areas requires shoes off, so it's best to get something safe and warm but easy to put on and take off as well.
Ice cleats(aka ice claws, snow grips, shoe grips, ...) like these small ones are a great addition as they have metal cleats to help grip while you're on icy roads and they're easily removable when you're about to head indoors. There are many variations but something like this is all you'll need, and the location should be right on the ball of your foot(the big bone behind your big toe) as most of the pressure when you walk will be on that spot. Buy them on Amazon, at outdoor gear shops in Japan or your own country, or sometimes they can be found in convenience stores in Hokkaido alongside hand warmers. In Japan, you can ask for "靴の滑り止め"(kutsu no suberi-dome: slip prevention for shoes)
For a photographer.
As someone who carries the traditional DSLR, and the suit of lenses and gear that comes with it, there's some ways to make everything easier.
I usually carry something far bigger with more sockets(6-8 at least + 5 USB sockets) when I'm traveling with friends and everyone can plug in at one spot without worrying about travel adapters(with a power bar, you'll just need the one adapter), but something this size is probably more suitable. Make sure it has both regular sockets and USB plugs (helpful for recharging battery banks, pocket wifi device, smartphones, tablets, ...)
The choice of a large suitcase is mainly because of one item, the tripod. While if i don't carry a tripod on a trip, i'd still use the large suitcase for carrying purchased items back, but the size of the suitcase depends on the length of my tripod. Typically the rollers that i use, the tripod(without the ball head) fits exactly the length of the suitcase, right down the center between the two plastic spines that holds the retractable handle bar's sheath (add pic later).
Orico is a good brand with lots of choices, and a tighter frame than the monstrous Belkins. Although Belkins have surge protection, which are better for countries with less stable power.
Remember that extra sling bag that goes into the suitcase with all the cables? That's also where the ballhead goes, unless your backpack has room for something that heavy, then it definitely goes in the backpack. Slightly overburdening the backpack is much less costly at most airports than allowing the check in suitcase to go overweight, especially in places like Japan where if they say it's 23kg, they'll ask you to repack at 23.2kg to get rid of the extra weight or pay a hefty surcharge, while in the mean time, my backpack is weighing in at 15kgs and does not get checked for weight or size anywhere in Asia so far. North America I'm not so sure, but I've been told it's the reverse, where they're more stringent on the carry on items, but a bit more lenient on the check in luggage. Please feel free to correct me if this is not the case for your local airport.
Typically from empty suitcases to being fully packed now takes me about 1-2hrs max, and most of that time is to roll my clothing neatly and calculating how many changes of clothing I'll need, depending on whether I'll have access to laundry (as there's usually little to do in rural Japan after dinner). Once you have your system down pat, packing should not be a hassle. Make a checklist if that makes it less likely to forget items, but the most important things are just your passport, money, and perhaps camera gear. Everything else, is easily replaceable when you're in Japan.
A cheap sling camera bag like this will protect your cables and adapters from being knocked around, and keep them all in one place during flights, and they make for good day bags, or an overnight change of clothes along with your basic toiletries bag.