Just go do it.

Yeah, I know... easy for me to say it.

But the thing is that there's only one way to really beat stage fright: Get up on stage in front of a lot of people and speak, play, sing, whatever, and as time goes by you'll become more and more comfortable. Everyone so far has given good advice. Just absorb yourself in the performance, go have a good time, and you'll come out OK. And remember, no matter how badly you may flub something (hey, it happens), just keep going don't let it stop you: If you screw up a cord or miss a note about 80% of the people won't notice, the other 20% are musicians and know what you're going through. But if you stop everybody will know something is up.

The bottom line: Go have a good time. If you're having fun you're less likely to mess up, if you do you're less likely to care, and the crowd will be more into what you're doing.