Instead of focusing on big things that could go wrong, focus on small, concrete things you can do to make your presentation better. That way when you're speaking and you wonder, "are they getting it?" or "am I being dull" or whatever, you'll have something you can do to fix that problem. I also spend a lot of time "rehearsing" various parts of lectures 'n whatnot in my head. It's not really rehearsing--I don't tend to go in order, and most of the stuff I think through I don't actually say. But, I think coming up with a handful of ways to cover a topic makes speaking easier. Nerves never go away; even after lecturing for a while I still get nervous right before classtime. That's something that you just have to practice getting through, I think. Once I actually start talking I usually relax and things get better.