I came across an article describing the process behind the curtains of Apple’s keynotes by Steve Jobs.
I always found his presentations very good. He surely manages to get, and maybe even more important, keep your attention. Ok, sometimes he exaggerates a bit, but if you want to draw people’s attention or convince people of your opinion, I suggest you have a look at one of his presentations, and start learning. A good candidate might be yesterday’s Macworld keynote, where Steve introduced the first Intel-based Mac. You might also have a look at last year’s WWDC keynote (where Steve introduced the move to Intel, and had to explain why this was necessary and beneficial).
The article discusses what I already suspected: the presentations are very carefully planned and prepared. Having it all in your head in advance allows you to concentrate more on the message you’re trying to transfer, and leaves room to improvise.
A common mistake when presenting is putting far too much content on your slides. To the audience, it’s like trying to read an article in a 9pt font with no paragraph divisions. After a while, you just give up.
A lot of tips and tricks about giving presentations can be found at Presentation Zen. It also features an article comparing the presentation styles of Bill Gates and Steve Jobs.