One of the panel topics at the convention I attended last weekend was self-editing, and since that's also a phase I'm currently in, I thought I'd talk about it here.
Conventional wisdom is that you really need a second set of eyes to look at your writing, and to some extent that's true, but it depends on your skill and experience level. I went to journalism school and have worked as an editor, so I'm probably a bit ahead of the pack, but this is a skill anyone can learn. You can find books on editing and grammar to learn all the rules (and when you can break them), and reading good work also helps you internalize the way language should work. When you have other people review your work, you can learn from what they point out. I pay attention to my copy edits so that I won't repeat those same mistakes. Even if you do get someone else to read your work, you should make it as perfect as you can before you send it to them, so you need to learn to edit yourself.
For this phase of editing, I'm not talking about working at the story level -- fixing plot holes and the like. This is more like doing your own pass at what a copy editor would do for your book if you sold it. I like to do this kind of editing in a couple of passes because they require different kinds of reading. First, there's what I think of as the continuity pass. This is a fast read, where I try to read as much as possible of the book in one sitting, the way a reader might. That helps me spot any variations between what's said at the beginning of a book and at the end -- does anyone's eye color suddenly change, does the house move down the block, etc. It's also a good way to spot jokes or descriptions that may get repeated. When months have passed between writing chapter one and writing the end, I may forget what I've written, and a joke or a description that seems really apt may come up early in the book and then be used again later in the book. Oops. A fast read will also reveal any obvious pet words. You may use a word only once a day or even a week when you're writing, but that can still be too many times when you read the book all at once.
And then it's time to proofread, to make sure all the words are right. Some people like to print out the manuscript at this point. I'm used to editing on the screen from my old job when I was editing the work of people around the country, so I do that, but I change the font so that the book looks different to me. It puts words and sentences at difference places on the page, and you'd be amazed at the difference that can make. Then I read the whole thing out loud, word-for-word. That keeps my eyes from just skimming over the screen and forces me to notice if there's a word left out or a word that's wrong. Your brain is really good at filing in blanks and reading what should be there rather than what is, and reading out loud forces you to see what's there.
Ideally, you should try to take a break between passes to get the book out of your head and make it fresher, but deadlines don't always allow that. When you're trying to sell a first book and don't have a deadline, it's in your best interest to take your time and put the book aside between drafts so it's like reading a new book when you get back to it. Even when I write a blog post, I go get a cup of tea or read a few web sites before I go back and edit it.