I saw Guy present at the PalmSource show in 2004. It was a really funny talk about what we learned from the dot com bust. One of his big things is doing top-ten lists in presentations. They're fun, they have a natural structure, and you don't end up reading off the slides but talking about what each slide means. Guy's got a blog now, and this post gives his tips about doing an effective PowerPoint presentation. The 10/20/30 comes from his main points:
10: no more than ten slides
20: present the whole thing in twenty minutes
30: use 30 point or larger type to keep yourself concise
I think this is very good advice for most kinds of presentations, although you may want to be sure you provide a way to give your audience more supporting information outside of the presentation if they're interested.