A proofreading trick I use sometimes: Touch each individual word as you read it (with a physical pointer like a pen or with the mouse pointer). For even greater accuracy, read aloud while you're doing this.

A proofreading trick I have not used much: Make a printout and read it upside down.

Edit:

Proofreading with a bigger font makes it easier to catch stuff like "assume" versus "assure". A serif font makes the difference between "I" and "l" visible. In a fixed-width font (e.g. Courier New) all the characters are the same width, so spelling and punctuation errors are easier to see (compare "rn"/"m" versus "rn"/"m").