I get the worst stage fright....
...Still.
When it was easily classified as 'stage fright' I could get advice from people, and I found that just going out there, and doing my thing, like JohnCheng mentioned, worked out really well.
Kinda.
I went out there with the notion of 'I'm as good as I'm going to be at this second in time, If people don' like it, they don't have to listen. luckily most people are really supportive, even if you're really not that good, as long as you yourself are proud of what you've done.
Seriously, no one's going to make fun of you.
If anyone does, they suck more than I could ever possibly imagine and (as corny as this sounds) really aren't worth your time.
So that's good.
Now the next problem is what poke'mon stage fright might evolve into.
Adrenalin-mon!.
Have you ever heard that athlete's hearts start pumping faster, getting all ready and exited even before they're about to do their athletic thing? The same is true for performers.
They get soooo used to that adrenaline and endorphin kick that they get it, even if they're not 'afraid.
Like: I'm not worried or anything, I'm just so exited I shake and can't breathe. That's really not good, especially since I love performing, and am so exited, that I don't think it totally can go away.
No amount of performing experience really makes it go away. I usually find that, As long as I stay up there, I always feel better after the first or second song. The same's true with normal stage fright. You find your stride, or become absorbed.
That's why, If I have some performance at a con later, I'll go to Karaoke and sing everything I know to get the pointy pang of 'OMG' outta me. If there is no karaoke, or if it's a different type of performance, I'll just start singing in front of the world for a little while.
The world understand, and will support you.
Most random people like it.
Getting your friends to do whatever with you during 'cool down time' is even better.
Helps you to not take it so seriously.
Also, if I can get in the back and do my thing before the performance, I'll do that too. Doesn't help tons, but a little. In a sense you're 'singing/performing yourself there.