Among the most common mistakes people make with slideshows is adding too much text, followed closely by formatting the text in such a way that it's difficult to read. Let's look at each of these in turn.
The most important bit of text on a slide isn't the set of bullet points; it's the title. The title is the bit people will absorb, and it's the starting point from which your speech should progress. The bullet points are, at best, reminders of a few of the supplementary points that you are making vocally. Remember, when people are reading bullet points, they aren't listening to you. So the best text-based slides will contain only a few words that can be immediately absorbed by the audience allowing them to devote their full attention to your speech.
Connected to this is typographic formatting. For short bits of text, you want text with a high degree of legibility. A good rule of thumb is to choose a simple sans-serif font similar to the sort of thing used on road signs, magazine titles, or anywhere else where someone is expected to immediately absorb short bits of information. Avoid italic, fussy typefaces with decorative scrolls and twiddles, and fonts with unusual degrees of compression or breadth. If in doubt, stick with something simple like Helvetica. Never write titles in capitals; capital letters slow down reading, so use them carefully and only where essential.
Color is critically important. Not everyone in your audience will see colors in the same way you do, and different projecting devices all have varying degrees of color fidelity. So stick with high contrast color combinations. Black text on a white background happens to work best, so a safe default is to stick with this combination or something very similar, like dark blue on pale pink. Light text on a dark background (like the infamous yellow text against navy blue beloved of inept PowerPoint users) isn't a good choice at all because the eye doesn't respond well to this balance of colors. Look around at computer screens and printed pages—everywhere text is expected to be legible and easy to read, it's dark text on a pale background.
Use the Fonts and Colors palettes to select and adjust the typography on your slides. Step away from the computer and look at your slides to confirm that the typography you're using works well at a distance.
Figure 6: Choosing the right font is important. A serif font (like Didot, top left) can appear fussy and difficult to read from a distance. Ornamental and all-capital fonts (like Papyrus, top right) are even more difficult to read. The wrong color combination does nothing to help an otherwise highly legible font (Helvetica Neue, bottom left) but switch the same font to a high-contrast color combination and it becomes easy to read (bottom right).