I don’t think there is really anything you need to do to prepare for living in Japan. Before I came, I took some basic Japanese courses, but when I arrived I found I couldn’t understand anyone besides Japanese teachers. I don’t particularly regret not studying more before I came here though.
After living here nearly ten years now, I wish I was more fluent in Japanese (I can have conversations, but don’t speak well). Perhaps either going to intensive Japanese school or working for a company where I had to speak Japanese would have helped in that regard. But if I made different choices, they wouldn’t have necessarily been better, just different.