There is only one rule actually.
But first, my top five spreadsheet rules were noted here earlier, and I was going to follow with similar principles of Powerpoint design. However it looks like Seth Godin has just done it. He actually lists nine, but I'll quote five of them below (with some slight paraphrasing):
1. Keep it short. Ten minutes. The rest of your time should be answering questions raised in those ten minutes.
2. Do not let people to take notes. I don't mean tell them to stop writing, I mean they should come away with a crammed mind, not a crammed notebook.
3. Cut down the words. Words belong in memos (and blogs). Powerpoint is for one big picture.
4. Be clear what it is that you are selling. It might be an idea, or a budget, but it's still selling. If, at the end, I don't know what you're selling, you've failed.
5. Don't use Powerpoint at all. I do like to start with a clean whiteboard, then gradually build up a flowchart.
So I can simplify all those steps into just one rule. If the subject matter allows it, to cover your entire presentation, build precisely one slide. That's all.
Those are not just random blogwords, I actually try to follow that rule when presenting. Most of you do not.