Every native-level, non-ethnic Japanese speaker I know got there with two tricks:

1) Working for an extended period of time in a (mostly) non-translation related position at a Japanese company

2) Hanging out near exclusively with Japanese friends

And a third that really helps with #2:

3) Having some shared hobby with said Japanese friends—whether it be mountain climbing or sitting on a couch playing video games, something that gives them something to discuss other than gaijin-ness.