Read his article in the New York Times here - it has great insights on speaking and communicating. Written in his own humorous style, as a comedian he rightly emphasizes humor. And he tells stories - well. If only politicians could do that.
Although I think he misses the mark with Barack Obama (Obama is a great orator but he does NOT use the teleprompter well) Cavett gives a lot of tips and techniques from a pro that are very useful to any communicator.