The main thing that ruins presentations as an audience member and judge, besides lack of knowledge (ie reading verbatim) and preparation is speaking too fast and mumbling. It is paramount that you open the presentation well so that the rest will flow smoothly. Be aware of everything you are doing (fidgiting, looking down, wobbling/kicking your feet) and speak slowly and outwardly. If you don't know where to be with your hands or feet, JUST STAY STILL, it is always better than wandering aimlessly with your hands or feet. If you don't want to look at the audience look just above one of their heads and scan the room that way. Practice in the room beforehand if you can, even if there are no people. It will do wonders.

And having a presentation you enjoy always helps too.

B