I'd just like to reinforce a few of the points above:
1. (Guest Giles) "Nerves are selfish" - do you know, that's the first time I've heard that since when I was a shy child and my Dad, a wise man, told me that shyness (and nerves) is a kind of vanity. It helps me if I remember that I'm only one of a number of people performing and I'm no more important - and probably no more nervous - than anyone else. And this leads to:
2. (Anne) Think of the audience and what you're offering them, which is NOT yourself but your song. If you concentrate on the song and what it has to say, rather than on your performance, it will come across more naturally - and it'll be a lot easier to remember the words.
3. (Hamish) Don't apologise or put yourself down before you even start. It just makes the audience edgy, and some of them will wonder why you ever got up to sing if you've got a cold/aren't sure of the words/can't think what to sing, etc. Just tell them you've got a great song for them, and then go for! If you enjoy it, there's every chance that they will too.

Good luck,
Marje