With an impromtu speech, simply have a conversation with the audience. I say conversation to the effect that you simply talk to them using a casual voice, asking them rhetorical questions that make them think. This is a good method for persuasive speeches, an example would be stating your side on a political issue. This way you are at ease with the audience, and as they listen, you seem to "just make sense."
With a planned speech, it is important to make eye contact with the audience. Hand motions can also help hammer in a point, but don't get John Kerry with it. He usually made one too many hand motions during the debates, as SNL jested over. Then again, remember Dubya's facial expressions. Remember to smile when you need to, have a serious look when you need to, and be aware that the audience is always looking at you...so no grimaces.