The scifi fan site io9 is not only a great place to keep up with your favorite geekdom, it also has great and useful articles for your career, like this one: How Cognitive Science Can Improve Your Powerpoint Presentations
The four basic points are:
The Goldilocks rule
Include only the data needed, nothing more. Anything else is distracting from the message.
The Rudolph Rule
Do something to make the important data stand out- a different color, size, seperate it, anything, just make it different than the rest.
The Rule of Four
The human mind can visually capture about four pieces of visual data at a time- so keep your slides broken down to four batches of data or less.
The Birds of a Feather
Organize like information in groups. It will help people process those groups of information like a single piece of information, better enabling them to intake the information in question.
Great stuff. Try it in your next presentation!