Recommendation: How To Speak by Patrick Winston

The lecturer should give the audience full reason to believe that all his powers have been exerted for their pleasure and instruction.
Michael Faraday

The following video by Patrick Winston on «How to Speak» has some interesting tips on how to speak — in lectures as well as job talks. And personally, well, I might be actually using chalk in the next semester.

You have to match his suggestions to your personal style, but still, highly recommended.