Don't just read what's on the slide. That's the classic boring PowerPoint presentation. Your audience knows how to read, so point out what's important, give context for the material on the slide, and so on.
Engage the audience. Make eye contact instead of facing the projection screen or looking down at note cards. Engage the whole audience, not just the instructor.
Speak clearly. Speak loudly and distinctly enough that people can hear you without straining. Avoid trailing off at the ends of sentences or speaking too fast.
If you run out of time don't race through the rest of your presentation. Skip some details and go to your conclusions.
Rehearse! Give your talk to friends and ask them for suggestions.
Devote each slide to a single theme. Put a title at the top of each slide that summarizes the point you are making.
Use the fewest words necessary to make your points clearly. Your audience does not have time to read and understand detailed explanations.
All your text and graphics must be large enough to be legible to the whole audience. The default label size in plots (especially Excel) is always too small for a presentation so remember to make your labels larger. Likewise images extracted from another source often need to be retouched. In short - if something isn't important enough to be legible from the back of the room then get rid of it.
Never use a slide filled with dense, small type, whether words or data. This is guaranteed to lose your audience's attention.
Be careful with slide backgrounds as they sometimes make your text hard to read. Avoid garish color schemes and hard-to-read color combinations (such as one light color against another or one dark color against another). Consider using a "minimalist" approach that focuses on your content instead of visual frills.
Rehearse! Make sure your materials are readable and free of typos or other embarrassments. Colors that look fine on your monitor can look much worse when projected, so rehearse in a room with a projector if possible.
Ensure in advance that your presentation is viewable. If possible, test your presentation on a room-sized projection system beforehand. Embedded animations can be especially tricky.
Rehearse! Work out any technical kinks (like graphics or animations) ahead of time so you don't have to apologize to your audience for something not working.
Did I mention that you should rehearse? You really should rehearse.