Russian was my first language, then I moved to Germany (age 5) and starting at age 6 German became my 'main language', in that I started thinking in it and being more fluent in it than in Russian, which I only used at home. At age 16 I set myself a challenge: think only in English! After some time, it became my thinking language, although maths were still done in German (German school, German university).
As the others, when talking in a specific language I think it, too. Recently I started forcing myself to count and do maths in English as well, and now it became effortless. At some point, I found thinking in Japanese to be quite pleasant, but as my vocabulary is rather limited I switched back to English. Really, my reasons for thinking in a particular language are convenience and aesthetics.
What I am dreaming in? Just this night I was dreaming in English; I can distinctly remember English phrases that I said.
When making personal notes, I write them down in English. Also, whenever there is a choice between English and some other language (web sites, books) I prefer the former.
Even though I've only ever been once to England for one week and have no English-speaking friends/relatives, it became my most used language. I just like it way more than my other two alternatives and it's much more practical.
I am 21 years old now and speak fluently in German, Russian, English and to a limited extent in Japanese.
Now I use Russian to talk to relatives and my girlfriend; German to my friends, acquaintances, generally people around here; English for consuming media (movies, books, internet).