How did we learn our mother tongue? Easy; through necessity and immersion. A need-to-know basis is one of the best teachers. You basically force your brain not to forget. When it comes to learning Chinese (or any second language), there usually is no imminent "need-to-know". You must become creative. Put yourself in situations that REQUIRE you to learn. Of course, the best way to do this is to spend some time in China or Taiwan where Chinese is spoken (preferrably without hanging around people who speak English, but if you can't avoid them, try to practice with them). Here's a few ways to immerse yourself in Chinese:

1. Watch Chinese Television

2. Read Chinese Newspapers or Other Literature in Chinese

3. Listen to Chinese Music

4. Eat at Chinese Restaurants

5. Label Items in Your House Their Chinese Equivalents

6. Carry Flash Cards With You

7. and by all means, Talk to Chinese People!