One should not be stiff, but slouching suggests a laziness that could possibly affect the audience's recpetion of your argument. Third, smile before you say your first word. Even if your subject matter is deadly serious and horrific, a smile will establish a rapport with your audience that can be built upon. Actually, one should think of something amusing as one walks to the podium so that the smile is genuine and not forced. Fourth, disguise your nervousness.
Every speaker is nervous.
But if your nervousness is apt to manifest itself in shaking hands, then grab hold to the sides of the podium. If you voice quavers, then speak a little more loudly and deeply. The signs of nervousness will go away after the first minute or two as the presentation of your argument takes control of your mind and body. The human body is capable of about 700,000 different gestures that can be used to communicate meaning. Gestures are the single greatest asset of a speaker, but they must be used sparingly and judiciously or else they overwhelm the content of the speech.