
Look, we are naturally blessed with the best presentation tools in the universe - the human face, voice and body. They are just fantastic.
But - the human eye is drawn to light. It's why we look into the fire, sit around the TV, and stare out at the sparkly ocean.
So if I'm using every part of my being, every muscle in my face, body and voice-box to sell you my idea, then the last thing I want is a large glowing rectangle behind my left shoulder.
That area of light will mesmerise you, it will suck out your attention through your eyeballs. Look at the image at the start of this post, who would look at the presenter's face or body language?
This is why you should think very carefully before deciding to use Powerpoint/Keynote or whatever.
(If I use an aid at all, I prefer a flipchart when I present. It is human-sized - I am pretty sure I could beat one in a fight. With preparation it can do almost anything Powerpoint can do, and most importantly it does not glow.)
For bigger spaces, you are going to need a bigger visual aid, I admit. But be very careful. One of the few people who gets this nearly right is Steve Jobs in his famous Apple product presentations. Looking at these from a theatrical point of view you will notice that Steve's huge Keynote screen only dominates when he wants it to dominate.

The result? We are looking at Steve, not at the screen, while he is enthusing on his ideas.
The message is simple: always remember the presentation is YOU.
So avoid projected light if you can - or get yourself projected up there.
PS: If you really can't avoid using the Big Screen, then only use it when you absolutely have to, concentrating on bare bones facts (like Steve) and emotive images (like Seth Godin recommends). And as soon as you are done with each slide, TURN IT OFF!