Look the part: for a serious speech look like you are the speaker and you do know what you are talking about. Your clothes give you credibility so leave the trainers at home and put on decent footwear.
For an entertaining speech you could give some clue that this is supposed to be funny in an article of clothing or head-gear. Although a speaker is not an actor, looking the part pleases the audience and helps them to focus on the theme you are presenting.
Science Makes Sense Creationism 