No matter how hard you try or how many times you proof your writing, it's almost impossible to fix all of the typos. So what can you do to correct the final 5% of your errors? I suggest that you have your computer read it to you.
Computers don't make mistakes—people make mistakes, even in writing. I have found that Text-to-Speech software can greatly improve your chances of finding the elusive typos that you just can't seem to ever see with your eyes—things like word transpositions, similar words (like than and that), and even missed punctuation. If you're a good reader like most writers are, your brain tends to fix these problems when you see them, so that the writing makes sense in your head even with the typos—which is normally a good thing, but not when editing.
Reading aloud helps to add another sense to your proofing.
I just finished the final edits of a middle-grade novel that I thought was pretty clean. I used a program called TextAloud to read the story to me aloud as I followed along in Microsoft Word. I was amazed at the simple typos I found, as the computerized reader pronounces words exactly as they are typed and makes the errors obvious. Even things like a missing period became apparent, because the voice doesn't pause for normal sentence breaks without appropriate punctuation.
I highly recommend using an audio text reader during one of your final passes through your manuscript—even if you think you've done your best to catch the typos, or even had your critique group look it over a couple of times.
I'm sure there are other Text-to-Speech packages available besides TextAloud, but I highly recommend this one—particularly the voice called Audry, which is a U.K.-accented female voice. I seem to be able to pick up on audio typos much easier with a U.K.-English computerized speaker. Unfortunately, extra voices often add to the cost of the package.
Try it! Obtain a trial version if you're not yet convinced. You might be surprised by what you hear.
(Disclaimer: This is not a paid advertisement for Next-Up or TextAloud. I was not compensated in any way for mentioning their product. Other text-to-speech products would likely work just as well. By mentioning TextAloud, I in no way imply that it will work for your needs and requirements.)