Recently Eric Lippert wrote about some presentation tips. I started to write a comment, but (as usual) the comment got so long that I decided to make it a post instead. You should read his first though.
My additional tips:
Turn up early to sort out all the technical stuff like how to turn the projector on and the lights off :-). At big conferences like TechEd they will do this for you, but you should still turn up early to give your machine enough time to boot, etc.
Don't talk too fast. Slow is bad, but fast is worse; at least you can understand a slow person, even if they bore you. This is very important if you are speaking to an audience that may not share the same native language as you.
Practice your demos 100 times, and then 100 times more. Make sure your machine is "clean" before going on stage -- it's best to have a batch script or some other automated way of getting your machine into a "known" state for the start of your demo script. VirtualPC works well here, too.
Turn off all other applications like Outlook and Messenger. You don't want them popping up alerts, or crashing, or using your CPU while you are presenting.
Know your stuff back to front, and be confident that you could give the same talk on a minute's notice without any supporting material if you had to (well, maybe a whiteboard if you need to draw pictures, but the slides should not be a mandatory part of your talk). This one generally only comes with time...
Less is more, despite what James Cameron says (remember James, if you're reading I still wanna do lunch). I remember (despite my efforts to the contrary :-) ) my early TechEd and PDC talks about JScript 5.x and JScript .NET. There were so many cool things to talk about that I just had a grab-bag of random features and other junk that I am sure the people who suffered through the talks didn't really get anything out of it. Decide on the one thing you want your audience to take away from your talk, and focus everything around that.
Don't let the marketing folks "clean up" your talk because they'll do silly stuff like capitalise the first letter of every "sentence" in your code sample (so it looks like you can't write code), or they'll mess up acronyms that they think should be (TM)-ed, or they'll replace words with similar-but-just-different-enough-to-make-you-look-stupid meanings, and they always ruin your custom slide builds / animations. I always look at what changes they made, merge the "reasonable" ones back into my copy of the deck, and then present that version.
In defence of transitions and animations, they can be very effective when you are building up an idea from smaller sub-ideas or when you are trying to emphasis portions of a diagram, for example. One of the things Eric mentioned was not to surprise anyone, and it can be better to build up (eg) a complex architecture diagram layer-by-layer and introduce each piece separately than to throw the whole thing up there at once and lose your audience. Now it's true that 95% of the animations do suck, but simple things like Fades, Appears, Dims, and the occasional Shrink-N-Grow come in handy. Don't go overboard!
And finally... Arial sucks, dude :-) It was cool in Windows 3.1 after the horrors of the bitmapped Helv typeface, but now it just looks plain ugly. Pick any proportional sans-serif font you like, just don't pick Arial! :-) Personally I use Tahoma or Verdana most of the time.