12-15 lines of text per transparency is plenty. It's easier to keep focused on someone who's moving than on a motionless talking head. Hand gestures are also good. It's possible to overuse these devices, of course. Simply crossing from one side of the room to the other every three or four minutes is probably enough.
Monotones are sleep-inducing.
Since it's possible to speak in a lively, animated manner without changing pitch, many people don't realize they have this problem. Get a trusted friend or colleague to listen to your delivery and give you honest feedback. Even better, tape or videotape yourself and check out how you sound. Face the audience . Speak from the gut, not the throat.