This is so ADVICE from past and current JET's in Oita prefecture. Please remember, everyone is different and this advice may or may not apply to you. You'll also see that some of the advice will contradict the 'Bring' recommendations
Shampoo isn't necessary at all
Leave your game systems at home and go do things. Do bring a 3DS so you can stuff on trains though. Your college mentality, you are an adult now.
Winter clothes (they are easy to buy or have sent over)
clothesssss. Bring only like a weeks worth of winter, and a weeks worth of summer clothing. The rest you can just buy here (especially after you get a better sense of what is okay and not okay to wear at work/in public). There are great thrift stores and department stores.
Leave your revealing clothes at home. Though you can do what you want on your own time and part of this is cultural exchange, the stares might make you uncomfortable and you may find your students talking about you if they see you out and about in something revealing.
Cold weather gear unless you have the room in your case.
You only need one properly smart outfit.
More than 2 changes of work clothes
If you can fit into uniqlo sizes, bring less clothes!
Fan paraphenilia. It could break on the way here, and your new life will fill the clutter hole.
Art supplies etc. There are plenty of stickers etc here.