Whole books have been devoted to body language. This is not intended to compete with such texts but provides a few basic pointers on how to stand and the effect your posture has on yourself and your audience.
When you slouch on to the stage, staring at the floor, and mumble your presentation, no matter how fantastic the words are you will not get a good reaction.
Likewise, if you stand up proud, talk clearly with variation in pitch tone and speed and recite nursery rhymes to senior executives you probably will not sell any products.
When you are presenting stand proud with your stomach in, chest out, head up and look the audience in the eyes. This will make you feel more confident, and you will come over more confidently as well.
A lot has been written about hand gestures and moving around on stage when you are presenting. Rather than worry too much about what to do with your hands, my advice is to act as naturally as you can. Stop worrying about what your hands are doing.
The main thing to remember is that little things tend to irritate. So, try to avoid small repetitive movements. If you normally use your hands to emphasize what you are saying, then carry on and use them when presenting. However, remember that you are on stage so you need to be more dramatic than when talking one-to-one. Hand movements should start at the shoulder not the wrist or elbow.
Use gestures to help create a mental picture in the minds of your audience.
Feel free to move around the stage, but watch out that you are not constantly walking across the beam of your projector, if you are using one.
Another thing to avoid is tottering or walking up and down or side to side repetitively, or swaying back and forth on the spot. That is the type of little thing which can become irritating to your audience. If you find yourself starting to move, move properly, walk right across the stage. After all like it or not, it is you that they have come to see and the more you move around, in a purposeful way, the better it is likely to be.