ProofreadingYou probably need more proofreading than you realize. Your brain will correct things for you unconsciously as you read. If you go through the document a couple of times and don't see any errors, you should not assume that the errors aren't there.
You may want to make several passes through the document, looking for a different kind of error each time. Check your grammar. Check your punctuation. Make sure you haven't used an adjective to modify a verb ("He is...