Watch your timing, both while speaking and going through your slides. You don't want to go too fast, but make sure you don't go over your allotted time, either. (This is where practice comes in!) You might want to leave a few minutes at the end for questions.
Try breaking your slides into smaller chunks or segments, and make sure they flow. But don’t use too many slides, either; find a nice middle ground. If you look at all of them in the slide sorter, do they seem to flow logically without your speech backing them up?
During your presentation (on either PowerPoint or Keynote) you can press the "B" key on the keyboard, and the screen will go blank. This is useful if you need to go off topic for a minute, or you want people to focus on you while you say something extremely important. Just press "B" again and your presentation will reappear.