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Content

• Minimize the need for reading. Prefer visuals over text.
  (The presence of excessive text is often an indicator that the visuals badly designed.)
• Avoid technical jargon 
  
• Use active tense
  

Structure

• Lines should not be longer than 40 characters.
• 1 1/2 spaced text can be read 10% faster than single-spaced text.
• Justified text has no advantages over left-aligned text. If lines are short, the reading speed of justified text is 12% shorter due to the larger spacing between words.
• Centered text of several lines is very hard to read.
  
• Avoid poor contrast to background, noisy background
• Information units, particularly in help and error messages, should not be longer than 12-14 lines (plus possibly a figure). Use "more..." links for more detailed information.
• Emphasis should not be used very frequently.

Fonts

• Text in upper/lower case will be read about 12% faster than text in upper case only (Rehe, 1974).
• Serif fonts are more easily readable than sans serif fonts.
• Proportional fonts are more easily readible than fixed-width fonts.
• Do not use more than 1-3 different fonts and 1-3 different font sizes.