Every writer knows that the most time consuming, difficult process in writing is the rewrite. "Getting the Words Right" by Theodore A. Rees Cheney is by far the best book I have found on the subject. He breaks it down, making it easy and effective to murder those little babies. This is a book I find indispensable. Great for those small tweaks or whole chapter cuts.
If you don't own a copy, I suggest you get one. I keep mine between the "Strunk and White" and "The First Five Pages."