How we change what others think, feel, believe and do
See yourself as others see you. A simple way of doing this is to practice whilst looking in a large mirror. This is particularly handy if you are stuck in a hotel room the night before doing the presentation.
An even better way is to video yourself. Many cameras these days can be used to make excellent recordings. Balance the camera somewhere or, better, get a cheap tripod so you can position it safely at the right level. Set it going and then do your presentation to camera.
You can try this with just a few minutes of presentation, working on the way you stand and the way you speak. You might also find you want to change the content of your presentation.
Another variant is to put the camera to the side or at an angle so you can see different aspects.
I want to develop a career in public speaking. I know my subject and think I'm pretty good at it, so I do a video of me giving an actual presentation, just setting up the camera at the back of the hall one day. I am pretty appalled at what I see -- I wander around, turning away from people and generally seem to be talking too fast. Although I am disturbed by what I see, I am also determined to improve, so I keep videoing and keep working on individual aspects of my presentation technique. Steadily, I get better.
It can be rather odd watching yourself, and possibly quite disturbing. Compared to others, you may appear awkward and ungainly. In fact a part of this is due to primitive echoes such as of the mirror phase. When others watch you, you will appear much more normal.
If you watch yourself enough and also accept yourself as 'just human' then you can be more objective. The trick is to see your image not as 'me' but as 'just a person'.
In your normal presenting and conversing you will also have all kinds of mannerisms, from the way you wave your hands about to the common words you use. Watching yourself, you can spot these things and consequently work to address them.
Do note that just because you are aware you can control yourself. It can be enough cognitive effort to keep on track with the words, let alone think about what the body is doing, though with practice and attention you should be able to gradually improve.