Plus, if your thought won't fit on a file card, then it's too big for the computer screen also. Aim for punch. Don't write everything on the screen. Just illustrate the main ideas. Use graphs, clip art and photographs.
Most people remember what they see better than what they hear.
Don't overload the show. If the gist of a slide can't be grasped in seven seconds or less, it is a waste of time. Try splitting it into two slides that deal with the same subject matter. Don't overdo the fancy transitions and gee-whiz effects. This just draws attention away from what you are trying to say.