@Downix:
Relax, you'll do fine :-)
The secret is to concentrate on what you're saying and trying your best to forget about the crowd. I usually try to unfocus my eyes when looking towards the audience, or focus a couple feet above their heads (darkened auditoriums rule ;-) ). As long as you don't squint or stare at the ceiling no-one will be the wiser. This obviously doesn't work if you're a singer or an actor, but I find that for public speaking it works great.
