The slide content should focus the audience. There should be a clear purpose, a single main point. Use words sparingly. Use key words that focus what you mean. Highlight short lists with bullets.
Pictures are better than words.
Graphs are effective. Label graph axes, lines and bars. Use only one or two fonts. San serif typefaces such as Avant Garde, Helvetica, Gill and Optima are preferable. Use large font sizes so people in the back of the room can read the words.