Slide Transitions
PowerPoint offers over 50 different slide transitions. And I don't think I've ever used 45 of them.
I have one simple rule for using animation:
Animate only if helps tell your story
Sometimes, animating in elements on a slide or moving things around with motion paths actually does help convey your meaning. But a checkerboard transition from slide to slide usually says little more than, "Look at me! Look at me! I found the transition function in PowerPoint!"
If you're thinking about using the Wagon Wheel transition, I've got a rule of thumb: If you don't see Spielberg or Scorsese using it, don't use it for your presentation. Actually, I'll amend that slightly.
While 99% of video, TV and film edits are hard cuts, they have the advantage of sound, music and dialogue to link shots together. For presentations, I prefer simple fades from slide to slide to give a slight sense of movement and to prevent things from feeling like an old-time carousel slide show ("next slide...next slide...next slide...")
Rarely, I'll use a push transition as part of a trick for a larger animation effect. Rarely.
The soon to be released PowerPoint 2010 will offer new transitions. Yes, they will be cool (as they attempt to keep up with Appple's Keynote), but I beg you—practice some restraint...