From the Writer's Desk: All About Editing: Self-Editing

Self-editing is the final bit of work you should do on your manuscript before you send it off to your editor or your beta reader. Self-editing is also something you should be really, really good at--especially if you're going DIY the whole way. Compared to editing your own work, editing someone else's work is ridiculously easy, so if you're interested in editing for others, learning to self-edit well is one of the first steps you should take.

Identify your problem areas and focus on those.
I tend to abuse the em dash, the semi-colon, and the comma. Maybe you confuse your and you're, or you abuse the ellipsis. Whatever your weakness may be, focus on it. Scour your manuscript for instances of your weakness(es) and eliminate them. If you're anything like me, this will get frustrating and you will, eventually, learn to stop being so abusive of language.

Compare your manuscript to a traditionally published manuscript.
Especially if you are a "new" writer (read: a writer who has just set pen to paper/fingers to keyboard), compare your manuscript to one that has been traditionally published. If you write fiction, pick up a fiction novel. If you write non-fiction, pick up non-fiction. Really look at the published work. Do your quotations look the same? Do your paragraph styles look similar? Are your chapters roughly the same length?

Focus on the basics.
Spelling. Grammar. Use your word processor's spell check. If you use MS Word, give the sentences with the green squiggly a second glance, but keep in mind that MS Word is not programmed with the Grammar Bible. Speaking of the Grammar Bible, it's a good idea to learn the rules of grammar. You are not too old or too smart or too big to go back to third grade (or fifth grade, or whatever) and re-learn elementary English. Keep in mind that you can't effectively break the rules of grammar unless you actually know those rules.

Don't jump heads.
Nora Roberts is notorious for this. You're probably not Nora Roberts, so don't do it. Pick one point-of-view (POV) and stick with it. Many first-time novelists choose first-person narrative, and that's fine, but whichever POV you choose, make sure you don't slip into Author Narrative and tell the reader things your POV character can't possibly know. Don't mix things up until you're confident that you can do one style consistently and well.

Check your tense.
Examine your manuscript for consistent tense use. If you start off in present tense, don't suddenly start using past tense. It is entirely too easy to switch back and forth while you're writing, so be very careful.

Hardcopies are your friends.
The best way to edit your manuscript is the old-fashioned way: a printed copy and a pen that can double as a machete. If you can read your work out loud as you edit, that's even better. If you absolutely must edit on the computer, read backwards. Start at the end and work your way back, word by word, then sentence by sentence, then paragraph by paragraph.

For more self-editing resources, click the following links.