Relax, drink a lot of water before hand, and take lots of deep breaths. And breathe/sing from your diaphragm. And if possible, get your pitch right before you start. And breathe! Don't forget to breathe!

Singing is 99% breath support, remember.

Also, try not to pass out. And warm up as much as possible before hand.

If you know what you're going to be singing, practice it as much as possible before you actually try out. If you know the part you're much less likely to freeze up.

Try not to giggle nervously. A lot of people did that this year, and Mrs. Anderson didn't like it much.

Um.

That's all I can think of.