How to beat pre-game nervousness?

I play basketball. I am pretty good when I am at the park playing pick-up games. The only thing is for when it counts I don't do so well. Like when I play for my school I get so damn nervous that I will do bad and just nervous in general and I suck. I was wondering if anyone knows how to beat that and not be nervous or if anyone knows any good relaxation techniques or good mental preparation for pre-game. Thank You!