1. Be fluent in Japanese.
2. Be knowledgeable in Japanese customs.
3. Do not assume that Japan is anything like your anime/manga series.
4. Be prepared to be treated like either a second-class citizen or a sideshow freak because you're not a Japan-born Japanese citizen. Even American-born Japanese are treated differently. They are a rather xenophobic culture. There is an even stronger stigma towards the Chinese and the Koreans.
Further reading:
A Cracked article from 2012 about being a white person living in Japan.
Japan is still a pretty nice place to
visit of course. Living there takes a lot of adjustment, perseverance, and patience.
My girlfriend has a friend that moved there last year; English teacher too if I recall. Everyone wants to be an English teacher in Japan apparently. They don't talk that much though the first couple months were filled with anxiety attacks and complaints. She seems somewhat well-adjusted now.