In recent months I have seen over 100 presentations in a more than dozen different spaces. Nearly all were, in one way or another, difficult to understand and follow, not because of the speakers, but because of problems with the projected slides.
The room is rarely completely dark, so colors and values are diluted by the light in the room - either from overhead lights or through the blinds. The latter is especially bad on a sunny day.
Projectors enlarge the image greatly, so you cannot count on very much detail in the text or images. Most visual subtleties are lost.
Many presentations are developed on a computer screen which is backlit, has a high resolution and good dynamic range - all of which are missing when the slides are projected.
The audience usually sees the screen from very different viewpoints - the rooms are not like a movie theater - narrow and deep - but like a classroom, they are broad and shallow.
The presenters only has the vaguest idea of what is can be seen by the audience because they are often facing them and looking back over their shoulder to see the screen.
If you can, preview your presentation in the place where you will be presenting it - so you can see how much information you planned to present visually will be lost.
Make sure you don't try to put more information up than can be easily read as projected on the screen and give the audience time enough to read it.
Work with black on white with the text and be careful with font size, line length and word spacing. This can minimize the loss in readability due to light dilution of the slides. (Or, perhaps, white on black, as Dave Stong suggested to me. Is it more or less readable to you when projected?)
If you use Arial, be sure to use a small amount of letter spacing (.01em) so the r and n don't run together and make an m (then yarn won't look like yam).
Don't use fonts with hairline serifs (common ones are Bodoni, Didot and Walbaum) because the thin serifs disappear when projected.
If you are using photographic images, taking them into Photoshop and increasing the contrast and saturation will make them visually more effective when projected.
If you are willing to take the time to learn how, make your presentaion in html and post it to the web. Then it will be easy to scale on the screen and available to anyone who wants to review it later.
Here are the slides for a talk I gave recently about color on the web. http://www.personal.psu.edu/
If you view it on a computer screen and then project it you can see how most of the color information available on a computer is lost when projected on to a screen. For instance, look at the reds with differing opacity and saturation.
Audience only got the points I was trying to make if they had reviewed the slides on a computer before the presentation. (They were available on the meeting web site.)
Let me know if you have any thoughts or suggestions.
16.11.10
Note - this site is designed to work on all platforms that support web pages, from smart phones to projection on a large screen. If you find any, where it fails, I would like to know. Thanks.