Body gestures, or body language, begins the second you are seen. Understand the importance this has regarding your audiences first impression of you as a speaker.
What you choose to wear is obviously a part of this visible feature. Does what you are wearing correlate with the theme of your presentation and what the audience would expect?
Wearing painting overalls while emceeing your best friends wedding isn't going to score you any points. Likewise, wearing a tuxedo at a turf grass irrigation assembly demonstration will do nothing for your credibility either. Dress appropriately to what the presentation calls for. That's the first thing.
Secondly, whether you like it or not, you're the expert. For that presentation, you have been selected to speak. So, stand like an expert. I'm not advocating being all puffed-up and pompous. However, carrying yourself in a slouched and uncertain way is just as detrimental. Hold yourself as if this is the best day of your life.
As you walk, move deliberately. First decide where it is on the stage you want to go, then resolutely go there. Not the other way around. To shuffle aimlessly until you get somewhere portrays no strength, no conviction.
It's also very crucial, during the first 15 - 20 seconds of your speech, to remain as confidently still as possible. When delivering that all-important opening you must draw your audience in, you must be unwavering. This sets the stage for your credibility.
The more you move during this initial engagement, the more "clownish" you will come across as. It is virtually impossible to regain your full credibility when you don't allow the audience to bestow it upon you at the outset of your speech.
These are just a few tips on body gestures, body language, and stance, as it pertains to public speaking.