Read Duncan's post, then, here are some additional notes:
1) Practice. I feel like I haven't nailed a speech until I've given it 2 or 3 times. And I give it once or twice privately as well. (and practice while driving, everyone will just think you are using bluetooth)
2) Pick a person in each 'section' of the audience. Alternate looking at them. For large crowds, you are far enough away generally that it isn't creepy, and for small crowds switch people every 10 minutes or so. For large crowds, everyone will think you are interacting with them personally. Switch looking at people in the front and the back, if you can make them out.
3) Practice, alone, on the stage with the music, slides and gear that you'll be using later. If you can't, don't speak.
4) Clueless/Stoned AV people are the enemy and the enemy are legion. Remove them from the premises. (O'Reilly uses solid av people, for the record). Christian Bale was right.
5) Make your slides additive and in some ways, superfluous. I've given talks where projectors die midway through, I've tried to give slideless talks (which always will be marked down slightly by audiences, it is in their nature)
6) Take questions at the end. During the speech, any questions should be short and the answers shorter. Long q&a belongs at the end. Most people are coming to hear you speak, not the audience. Some are coming to give you crap, they can wait. Some have good questions, find them!
7) TV without an audience is different from TV with an audience which is different than speaking to 1000 people which is different than speaking to 20. The only thing that is constant is you and your knowledge, make sure you are up to snuff.
8) Don't lie.
9) Don't hype products that don't exist or won't for years. There were JavaMX talks being given at OSCON as far back as 2006. IT is a waste of time, but your audience will have time to check email.
10) Almost all panel discussions stink.
11) Wear clothes you like and you look good in.
12) Watch the video of your talk. Get ready to cringe. Sadly, people who do this often don't know enough to be self critical. Here's an easy tip: Watch yourself with your wife, sister or someone who can be gently critical. You'll find you are way more sensitive to your flaws when watching with someone whose opinion matters to you.
13) Pidgeon hole me and ask me to watch 10 minutes of a speech. I'm not gentle at all ;-)
14) Loosen up about time. Sometimes speeches start and end late, early, or more. If you end early, take a question or two, or let them leave, if you have nothing else to say or have bombed.
15) US Audiences are different than foreign ones.
16) Don't speak so damn fast. I'm totally guilty of this.
17) You don't need 300 slides. You are giving people siezures. Stop.
18) Speak everywhere, it'll keep you sharp. I've spoken to computer people, librarians, schools of every age range and level, elks lodges, etc. People are different and while you might resonate with computer people, you might find the rest are beyond you or your topic. That's life.
19) You -will- bomb, and hard.
20) Don't give speeches to people who don't really want to be there.
Rules of thumb:
1) 60hz at 1024x768 works on every projector.
2) Test your laptop shortly beforehand.
3) For non-coding slide,s every 2 minutes on stage can = 1 slide.
4) 2 private and one 'public' practice before going on stage might be enough.
5) Give speeches every week, if you can, but once a month or you'll start to backslide.
6) Have copies on usb sticks in pdf, ppt and, for free software people, odf. sometimes you will not be able to use your own gear or worse, your gear will fail. I also email myself a copy of the presentation.
7) Lapel mics work best on shirts buttoned up. If you normally leave button 2 open, close it for the speech.
8) Union guys? $20 goes a long way..
That's all I have for you right now. Have fun on stage!