/shameless plug from a fellow löver.
Use colors in /addition/ to other indications. Roughly ten percent of males are red-green colorblind and cannot easily distinguish colors that contain these colors (e.g., purple = blue + red = BLUE to them). Also, projectors are usually pretty crappy at colors.
"If PowerPoint ever automatically chooses a smaller font for your text, you have too much on the slide." -- somebody wise
Specifics: slide 34. Formula is (each item is a number of colors):
fill + text + lines - background
When we draw lines and fill shapes, we may introduce additional colors, or what feels as adding colors to a purely B&W picture. We could also have portions of text and/or background color-coded.
In any case, the author apparently counts the number of additional colors. He suggests the number should be small (no more than 3, see slide 35), in order for the viewer to keep the "color coding" manageable.
Presentation Zen Design from Garr Reynolds is another solid Powerpoint design guide.
I saw the slide and the "visual complexity elimination subroutine" kicked in. I'll try to fix that in a future version.
Another thing I experimented with for the first time is doing all the slides in html / css / js. The jQuery-Presentation plugin  works as advertised, and with css there was considerably less hairpulling when it came time to start tweaking layouts and positions. The content for each slide is written in markdown. You can see all the source for the presentation here .
(I can't wait for some web-based presentation software so I can finally ditch Powerpoint, but right now, with the extremely limited palette of animation ability on all of them, I'm forced to stick with PowerPoint.)
For papers, posters, and other static media, I use Inkscape, which I love. And every once in a while, for a system architecture diagram that I need to crank out really fast, I'll use Dia, which is much less flexible, but ideal for that task.
I try to use the best tool for the job though.
Two weeks ago I had to make a code-based introductory presentation and I used showoff instead ( https://github.com/schacon/showoff ) since it comes with syntax highlighting and the slides are just a text file. It took me maybe a couple hours to learn the ins and outs, but then it was easy.
I've heard that the Mac app is good, too, but I haven't used it yet.
Here's a dropbox link. It should be there 2-3 weeks.