This morning I presented at a community review for my manager, my second level, my third level, and my fourth level (one of the MS VPs). The review is about how our division is doing on community overall, and I owned talking about some of the things that we've been doing on the C# side (blogging, FAQ, etc.)
I was a bit nervous when I was on the way to the presentation, but I've found that if I do good walkthroughs ahead of time, once I start presenting, I'm focused on the presentation and I don't even think about being nervous.
Of course, the 8 hours I spent on writing slides and practicing helped.
So my guidance is
Never present in front of an important group of people without having practiced your talk at least twice.