Practice - The best presentation are by people who have practiced their speech over and over again. The more you practice, the less nervous you'll be, the more you can focus on the content of your presentation. And having the beginning of your presentation memorized will get you past your nervous jitters.
Know Your Audience - Are you presenting to a room of experts, a room of generalists, or a room of non-scientists? Cater your presentation to your audience. When in doubt, don't be afraid to explain the basics. It's typically always appreciated, even for seasoned scientists.
Be Yourself - Showing your personality in a presentation, especially by using occasional humor, will help your audience engage with both yourself and your material.
How to Give Good Presentations-- This guide from Harvard has such useful advice as what is expected at different kinds of talks (workshop, scientific meeting, etc) and how to format your powerpoint slides.