There is, and it's literally "use Japanese daily". How you do that is entirely up to you, but start speaking Japanese every day to people around you. Easy to do in a university or school setting, because everyone else is doing that too, harder to do from home, but that's where the internet and local classes come in. Use something like italkie, pay someone for home-tutor style conversation classes, find a Japanese cultural center near where you live and get chatting, hang out in Japanese chatrooms, just get yourself in there.

The simple truth is that you don't get to "highly competent" without "high frequency use".