Best way to develop your mathematical thinking.

Okay, I'll tell you a little about me.

I'm 16 year old, from Estonia. In math I think I'm in top 5-15% in my age. Well, okay that's fine, but I really want to be better.

I have no problem to solve the school math exercises, but I'd be glad to be better in olympiads. The problems over there are not normal and quite many of them which we haven't even learned about. So I was wondering, what would be the best way to develop myself to be better at solving mathematical problems which I haven't met before. Are there some kind of exercises that'd be good or perhaps I should do a lot of calculus everyday in my head to improve it. Which way is the most efficient, because I feel I could take a lot more out of my head than I'm taking at the moment.

I hope I chosed the correct forum to ask my question. I hope someone could answer me : )