I recently read Guy Kawasaki's "Art of The Start." In addition to being a good author/blogger, Guy was one of the very early Apple employees and more recently has been a venture/angel investor type where he has listened to countless Powerpoint presentations. Presumably because he is tired of seeing poor Powerpoint presentations, he spends many pages in his book talking about Powerpoint best practices. There were a few nuggets of Powerpoint wisdom among a lot of content about it that stuck with me a few days after finishing the book.
His mantra is that Powerpoint should follow a 10/20/30 Rule. There should be no more than 10 slides in the presentation -- very few people take away much more than one concept from a presentation, so all that other stuff is extra. The slide presentation should be designed to last 20 minutes, leaving room for ample questions/discussion between slides or after the presentation. Guy points out that the point of the presentation is typically to initiate a discussion. He says the font should be size should be no smaller than 30 (Arial font). Guy says that audiences read faster than you can talk, so that while you are up there talking, they are trying to read your slides and not listening to what you are saying.
He says that there are something like 60 animation features within Powerpoint and he recommends the less use of it the better. His advice is to use your voice/body to emphasize when a point is important, not some fancy Powerpoint trick. The only place he recommends using any of this is in going through bullet points on a slide, presumably to avoid having people read ahead. Speaking of bullets, Guy suggests that bulleted slides should have one point with bullets and only one layer of bullets (lest you violate the 30 part of 10/20/30).