Know the language?
It's about the one most important thing I can think of, to really know the language, it might help to find someone you get along with a trust in the area in Japan you plan to study, and then they can help you learn the local mannerisms.
It'd also help for you to know what Japanese food you like. It'll help you fit in a bit more too.
Those are the key bits I can think of, hope it helps. I've not studied abroad yet but am considering that opportunity when I go to university as some architecture courses offer time abroad.