You know those cool word illusions where it's hard to spot the repeated word?
Sometimes the position of a word on a page or in a sentence can affect the brain's efficiency at spotting typos. For that reason, it's sometimes helpful to switch to a different font before proofreading. It puts all the words in different places, and makes certain problems easier to spot.
I have a friend who reads manuscripts backwards one sentence at a time when typo-checking. Apparently it helps her brain disconnect from the running narrative so it can concentrate on the grammatical structure of individual sentences. I know other people who strongly advocate reading the text aloud.
None of which will substitute for getting a qualified second pair of eyes to read through the text, of course.
