At our November meeting, Andrew Doty of Editwright and Karen Tucker, the Comma Queen, gave a comprehensive discussion all about editing, and included tips on self-editing to make the manuscript the best it can be before submitting to a professional editor. That way, the editor can concentrate on making the manuscript better rather than fixing all the errors. This can lower your costs, too, since the editor's time is your money.
Joanna Penn just posted a helpful article of self-editing tips. (Note, she says you should still hire a professional editor!)
Writing Tips: 10 Ways to Fake a Professional Edit