
This article from WebDesignerWall offers some very simple, yet powerful ways to improve the look, readability and flow of text-based materials.
First and foremost, consider using different font styles, sizes and colors to emphasize (or de- emphasize) content and improve retention. Limit the use of bold, underline and italics. If you need to emphasize something try modifying the font color and font size especially for various headings and or text passages that you feel require emphasis.
For a more advanced discussion on the ways to enhance the readability of text, check out this presentation by Jeff Croft on Typography (pdf).