Why it's Hard to Proofread Your own Work
It's difficult to proofread your own work because you've already read it. Your mind knows what SHOULD be there and shows you what you expect to see. Take the sentence "Dead or alive, you're coming with me." Did you notice the extra space in there? In my opinion, punctuation errors are the hardest to catch because they're so small. It’s quite easy to miss an apostrophe or quotation marks.
People are always recommending you put your manuscript down for a while and come back to it later. This is to give yourself time to forget what’s there and re-read it later with a pair of fresh eyes. This is helpful because if you forget why you wrote something or what you were going for, you can judge whether it’s easy to figure out.
I do recommend Spellcheck and SmartEdit. While far from perfect, they'll give your work an extra layer of polish. However, there's no substitute for a trained proofreader. It’s an expense that I feel is well worth it—assuming, of course, you find the right proofreader. You know, someone dedicated to quality and professionalism. But where to find such a person?