Guy Kawasaki, the high-energy former "evangelist" for Apple (and a college classmate of mine, though I don't recall that we ever met) has what he calls the 10/20/30 rule of PowerPoint presentations: No more than 10 slides, no longer than 20 minutes, and not less than 30-point type! That may be a bit extreme, but I tried following that model for a recent presentation I did, and it worked pretty well. (I didn't check on the 30-point type, but I must've been close.)
I've really had to learn to keep my slides simple and uncluttered. Too much text is a killer. Also, I've seen too many people who just read from their slides. Boring!! Face the crowd and speak to them! (Of course, in my latest presentation, it helped that we have an additional screen in the back of the room that I could glance at to make sure the right slide was displayed . . .)
I am definitely no zen master when it comes to PowerPoints, so these suggestions and tips are all very helpful.