basicly you have to believe in yourself.
If you have practiced and rehearsed to near perfection (lol) then there's nothing to worry about. And as the others say , confidence will follow.

what i do to look even more confidant is to look out at the crowd while performing. The trick is to look at the crowd as a whole.

Never never pick spots or look at one person in the crowd. It'll just remind you that someone is watching your every note. Haha, its like they're staring into your soul. =S

Hope this helps =D