I was recently asked to participate in a poll on Goodreads.com about editing, specifically how authors get it done for their work. Responses to the question “Do you use an editor?” included did it all myself, had a friend or relative do it, hired a cheap or expensive editor, used a free website for authors to help one another, used a volunteer, or used software. Sadly, the most common answer that had floated to the top of the poll was, “No, I just did it all myself.”
Here’s the best advice I can ever offer a writer: DON’T do it all yourself. Find yourself a “no” person, someone both willing to read your work critically and that you’re willing to listen to no matter what they say or how bad it may seem to them (advisers are invaluable, see The Evil Overlord list for details on correct usage). Heck, get a team of volunteer readers. They may be grammar Nazis or just people who like to read, but they will see things you’ll miss no matter how many times you read it yourself. Sometimes it won’t be grammatical or a missing word but just not explaining things as simply or as thoroughly as needed. A paid professional is wonderful if you trust them and can afford it, but always, always, ALWAYS have another set of eyes go over your work any way you can get it done.
Also, if you can compile your work into a PDF (OpenOffice can do this for you), here’s a really cool trick: let Adobe Reader read it to you out loud (it’s a built in feature of the free version) and just listen. This will reveal a lot of mistakes you and all of your editors may have glossed over for a final edit.