One of the country's best presenters, Guy Kawasaki, followers a simple rule for his presentations. He calls it the 10-20-30 rule.
10 - Ten PowerPoint slides are the ideal number for a presentation.

20 - Twenty minutes is the longest a presenter should speak.
30 - Thirty point is the smallest font size that should be used on slides.
The key to the 10-20-30 rule is focus. By limiting the number of slides (10) and the volume of text (30 point) speakers are forced to focus on what's most important. They have to decide what the key points are that will get the message across. The 20 minute time limit reinforces this emphasis on focus. Decide what needs to be said and say it as clearly and succinctly as possible.
Next time you're asked to do a presentation give the 10-20-30 rule a try.