When I have been asked to address a large group its scary as hell! For me I agree with what Andy has said. Make sure you know what your going to say inside and out. Alway's stick to what you have wrote and try not to wander. I also heard that those time's when you want to say "ummm" to just be silent and gather your thoughts. Oh and lastly after about 2-3 minutes of being up there you tend to relax a little. Good Luck.