Practice, practice, practice.
I stopped formally learning Japanese after a few years, much like mdchachi. At that point, I continued to learn by reading, listening, and interacting with other people.
It's been over 10 years since I used Japanese on a regular basis, so I'm a bit slower than I used to be, and I have to concentrate a little harder. But I think it's like riding a bike. Once you've practiced enough to ride well enough, you never really forget it.