Guy Kawasaki has a very interesting post about The 10/20/30 Rule of PowerPoint. The rule says that a PowerPoint presentation should have ten slides, last no more than twenty minutes, and contain no font smaller than thirty points.
Guy writes that this rule is applicable for any presentation to reach agreement: for example, raising capital, making a sale, forming a partnership, etc. But I think few slides and large font is a rule that goes for any presentation. The slides are supposed to support your presentation, they are not intended to contain your presentation.
Suggested reading: How to Present and Pitch – A Guide to the Perfect Pitch
This was originally posted at another (now extinct) blog of mine.