I remember a few years ago that I had to get up in front of a bunch of executives with a microphone in a strangely-shaped room and give a presentation; I got scared and started sweating and gave a horrible presentation. There was really no need for me to present anything, but I had been told that I had to give it instead of deciding that it was the right thing to do. As Seth Godin says:
The best presentation might be no presentation.
If you're going to bother to do something, you ought to do it very well indeed. Otherwise, don't. Don't show up. Don't waste your time (or mine.)
Seth’s got a great a plan on how to give an effective and casual presentation by just being yourself — 4 plain and simple steps that you can follow to give a great presentation with little stress.