As travelhappy said, living in Japan is a fantastic way to learn the language, as long as you do try to learn it. I've heard of people who taught in English in Japan for a year without learning more than the basics (which most people could pick up after a week of effective study).
As an alternative to
living in Japan, staying in Japan
temporarily while learning Japanese is also great. Last year I holidayed in Japan for a month, the first two weeks of which were spent staying with a Japanese host family (very lovely people, can't speak highly enough of them) while attending a small Japanese language school. The school was called Asahi Nihongo (no, not the beer), and the whole thing was arranged via STA Travel.
I had the most amazing time and met some wonderful people; it's easily my most enjoyable experience to date. (well, my most enjoyable fully clothed experience
)
Ganbatte!
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If trees could scream, would we be so cavalier about cutting them down?
We might, if they screamed all the time, for no good reason.
--Deep Thoughts Jack Handy