This is some 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
Toiletries
Shampoo is perfectly available in Japan (even scent-free, sulfate-free etc.) or can be ordered from online stores like iHerb (there are also some Western stores in Japan, such as LUSH), you don't need to stock up or bring a load here. If you have curly or wavy hair, however, it might be worth bringing some more specialist hair products.
Unless you have particularly sensitive skin, you're sure to find skincare products you'll love in Japan, as there is a wealth of both Japanese and Korean skincare products available around the country (Korean skincare products will be less common in inaka areas, however).
Technology
Consoles - unless it's a Nintendo Switch or original DS (which aren't region-locked), the chances are you won't be able to play any games you buy in Japan on it.
DVDs/CDs
Clothes
Winter clothes (they are easy to buy or have sent over)
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!
Shoes
Winter shoes
MISCELLANEOUS
Fan paraphernalia. It could break on the way here, and your new life will fill the clutter hole.
Art supplies etc. There are plenty of stickers and craft items here.
Books. Bring a few favourites, sure, but too many will cut into your weight allowance.