--Take compact-size toiletries, brushes, shaving razors, etc. You can find travel size toiletries anywhere from pharmacies to grocery stores. Alternatively, you can fill small, empty bottles with product from your regular-sized containers of shampoo, body wash, etc. Pack all your liquid toiletries in a 1-quart size plastic bag. You will need it for the security checkpoint, if in your carry-on, but just as importantly, you don't want the liquids to burst out all over your clothes.
--Bringing electronics from the US to Japan: The voltage in the US is 120Vac RMS (60Hz), while the voltage in Japan is 100Vac RMS (50-60Hz). Essentially, this means that if you bring a hair dryer, it won't run as powerfully here in Japan as in the States.
--The key word is versatility. Bring a couple pairs of lightweight pants, a pair of exercise shorts, a few T-shirts, and one nicer outfit to wear if you go out somewhere special. If you only have one or two options, you will spend less time worrying about your clothes and more time having fun. Plus, if you like Japanese fashion, you can buy clothes here to wear and bring home.
--Shoes: The best shoes are the ones that are comfortable to walk in. If you bring gym shoes, wear them on the flight and pack sandals in your bag. You can bring indoor shoes/slippers if you like, but honestly, you will be just fine without them.
--Pack empty plastic bags to hold things like dirty laundry and wet towels/swimming suits in a pinch.
Last word on luggage: less is more.