The title says it all :)
I learnt English by taking a course (I forget at which grade I started the course, maybe at 4th grade). Aside from courses, I also got used to computers (back to those old days, there was no program using Indonesian). It helped a lot.
Also, sometimes I prefer watching DVDs using English subtitle than Indonesian, since, well, you know, Indonesian subtitling sometimes get so silly, doesn't translate some part of the movie, and sometimes it says Indonesian but it's Malay! Although, it doesn't appear to happen in original DVDs, but they are soooo expensive...
But, although I'm familiar with English, I use Indonesian as often as possible as my preferred language. See, this board uses Indonesian (except for the board names). My Gmail, Friendster, Yahoo!, etc. interface uses Indonesian.
@Exsharaen : I use the same way as you do. But I always use English as my interface.. Well, I tried to use Indonesian as my Yahoo Messenger language... and I get confused, so I switch it back to English. And another way that helped me learn a lot is reading novels that is written in English.
I took the course when I was at 2nd grade, and stopped at 9th grade - because they ran out of levels :p (I took Business English and S.A.T as my last levels there, it's crazy enough for a 9th grader to take a scholar's test...)