One of the key factors for me when editing is time — not time for editing, but time between edits. Often I’m blind to what I’ve written, so by leaving a piece for a week or longer before I read it again, problems become clear.
My first step is to edit on the computer, but I never discover half the ways I can improve my writing on screen. My next step is therefore to print out a copy and leave it for at least a day. I scribble all over the printed copy and allow another day or longer before I reread the piece yet again.
Once I feel I’ve done the best I can, I take it to my critique group who always find something I can improve on. This may sound like a painful process, but the end result is worth the sweat.
Near the end of a project, if I’m satisfied, I’ll read the entire manuscript. I didn’t reread my story set in Tanzania because I knew something in the plot needed improving, so left it for over a year while I worked on other projects. A couple of months ago, I thought of a solution, and now feel close to when I can say, I’m finished.
So my final advice: don’t be in a rush to send your work off to a publisher. Do the absolute best you can first.