Use suggestive graphical illustrations as much as possible. Don't shun graphical metaphors. Prefer an image to text. In my presentations I try to have 80% of the slides with images.
Do not put in the figures details you will not mention explicitly. The figures should be as schematic as possible (i.e. no overload of features).
Do not "waste" information by using unnecessary colors. Each different color should signify something different, and something important. Color-code your information if you can, but don't use too many different colors. Have high-contrast colors.
A few real photos related to your subject look very cool (e.g. real system, hardware, screen-shots, automatically generated figures, etc.). Real photos are much more effective during the core of the talk than during the intro. I hate talks with a nice picture during the introduction and next only text; they open your appetite and then leave you hungry.
For some strange reason, rectangles with shadows seem to look much better than without (especially if there are just a few in the figure).
Sometimes a matte pastel background looks much better than a white one.
Exploit animation with restraint. Do not use fancy animation effects if not necessary.
However, there are places where animation is extremely valuable, e.g., to depict the evolution of a complex system, or to introduce related ideas one by one.
Use strong colors for important stuff, pastel colors for the unimportant.
Encode information cleverly: e.g. make arrow widths showing flows proportional to the flow capacity.
Use thick lines in drawings (e.g. 1 1/2 points or more).