I haven't had time to read all the hints here yet (and it looks like there are some good ones and I'm bookmakring this), but I have to disagree with the first post, by Paul.

It's a BAD idea to put text on the slide that differs much from what you're saying, and even worse if that text contains information you're not saying verbally. What happens is people in the audience get torn between reading the info on the slide and listening to you, and we know from attention experiments in psychology that people can't do both, and will likely end up getting nothing out of either.

To paraphrase my supervisor, an esteemed psychology professor: you end up competing with yourself!

That said, I agree that you shouldn't have long full sentences in slides and then just read them. Instead, it's better to have something short on the screen that is the same as what you're saying out loud. Off the cuff ex: slide reads: "Theories explaining forgetting: 1) inhibition 2) blocking" and you say: "There are several theories that explain why forgetting happens, and I'll focus on two: inhibition and blocking. Inhibition means that access to an item in memory is actually weakened somehow. Blocking means that one item in memory is getting in the way of accessing another item."