I usually make my students give some sort of presentation every semester.  I also enjoy speaking at businesses and conferences whenever I get a chance.  So here are some tips that I use when speaking and that I provide to my students before their presentations.

1. Know your subject.  Be sure you are an expert on the topic you are presenting… Bear in mind that an expert is someone who knows more about the subject than 50% of the people you’re presenting too.  If you’re only a sudo-expert, you can become an expert with in no time.
2. Tell a story.  Be sure you have some case studies or good stories to add.  Always tell a story.  People relate and respond to a story more than just facts.  If you can provide a personal story you’ll have more impact with your audience.
3. Prepare, practice, and repeat.  Prepare your presentation and practice it.  Then repeat and repeat again.  Anticipate problems that may arise and have contingencies for those problems (i.e. projector doesn’t work, etc.). Be sure you practice out loud. I’ve found that I talk faster out loud than I do when practicing in my head.
4. Know your audience.  Get an idea of who you’ll be talking to and find a way to relate to their experiences.  If you’re talking to GIS folks, talk geography or maps, if you’re talking to software developers, talk debugging, IDEs, etc.  Relating to people on their level helps them relate to you and your subject matter.
5. Don’t say “hmm…Ah…”  Practice not saying “hmm… ah…” or words like that.  If you need to stop and think, stop and think.  Don’t add filler words.  Sometimes a pause can be used to add emphasis.  Also don’t apologize for mistakes, roll with them.  Most people won’t notice them if you don’t point them out.
6. Know your environment.  Visit where you’re going to be speaking if possible.  If not, ask what the room is like and be there early to get acquainted. It makes a huge difference in your comfort level if you’re familiar with your environment.
7. Relax.  What’s the worst that can happen?  Play out your worse fears and how you’ll recover from them in your head.  If you bomb (and believe me I have) you’ll recover in no time.  Understand that everyone there to hear you speak wants you to be successful and is rooting for you.