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.
What sort of lecturing do you do? How much does mastery of the subject help when delivering?
Variously: digital network design, calculus, C (beginner & advanced), discrete math, miscellaneous stuff-you-probably-should-know-as-a-person-who-programs-computers-sometimes. Mastery helps because you can answer "why": why is something useful, why would you care to study something (sometimes those two have different answers!), why certain algorithms give the results they do. Anyone can learn, say, how to calculate a remainder; mastery helps you connect remainders to other related ideas (cycles, clocks, symmetries of geometric shapes, cryptography...). Also, mastery helps you answer unanticipated questions and to come up with alternate explanations when your audience doesn't get what you're trying to say. It doesn't help when you forget that you know something--it's easy to gloss over basic concepts that have ingrained themselves in your mind so much that you don't even notice they're there anymore. This is not to say that I've mastered anything, or that 'mastery' is a final, well-defined state of existence...