Use the slide to show graphics that succinctly capture ideas and key points that you want people to remember. Your audience will best remember what you said in the beginning and at the end of your talk. Keep overlays to a minimum. They are awkward to use and most people spend too much time trying to get them aligned. Use technology appropriately.
Color can be invaluable, especially for displaying complex data, but it can also be distracting.
Speak in clear, measured tones. Don't speed up and slow down as panic about timing kicks in. Be aware of the time. Test run your talk with an audience, if possible. The more important the talk is the more you should practice it.