Realize that the following techniques are to be avoided. I have observed presenters that practice all of these no-nos and are never asked back.

Never look anyone in the eye. Scan the room with your gaze somewhere above the heads of audience members. I know that I have read this tip somewhere in the past, but I also know that if you do this, you will never make contact and interact with your audience. Yes, you will avoid seeing some who look bored and others who are dozing off, but you will never give the impression that you are speaking directly to everyone and never feel the encouragement and energy given back by those you look at