Read it slowly.

• Read every word. 
• Read what is actually on the page, not what you think is there.
• Use a spellchecker, but don't depend on it.
• Have others read it.
• Give a copy of the document to another person and keep a copy yourself. Take turns reading it out loud to each other. While one of you reads, the other one follows along to catch any errors and awkward-sounding phrases. This method also works well when proofing numbers and codes.
• Point with your finger to read one word at a time.
• Don't proof for every type of mistake at once. Do a proof for spelling, grammar, consistency of word usage, font sizes, etc.
• Keep a list of errors you tend to make and doublecheck for those.  
• If you are editing within MS Word, use the "track changes" tool to make your changes apparent to other reviewers (additions and deletions can be set to appear in different colors).
• Print it out and read it.
• Pay special attention to proofreading the headings. Headings are prone to error because proofreaders often don't focus on them.
• Double check fonts that are unusual (italic, bold, or otherwise different).
• Be careful that your eyes don't skip from one error to the next obvious error, missing subtle errors in between.
• Double check proper names.
• Doublecheck boilerplate language, such as EEO statements, department statements, etc.  
• Review page numbers, table of content and footer/header material for accuracy and correct order.