I've talked about revision before, but there's a difference between revision and editing, although I usually do both at the same time. Revision, at least in my opinion, involves changes at the plot/character level, whereas editing involves smaller technical changes, things on the sentence level instead of the scene level.
Along those lines, Sylvia Rochester has a post over at Heart of Louisiana that has tons of good editing tips for polishing up a manuscript that I'm not going to repeat here. They're all great tips and things that every writer should employ during the course of writing.
The thing that I have to keep in mind is that, while all these editing tips are useful, they are only useful in the contest of the story that I'm telling. So, for example, most of my main characters are teenagers. Teenagers have a certain cadence and method of expressing themselves that are unique to teenagers (as a stage of development), and then on top of that, there are layers of individual character expression that have to be added.
What does that mean in terms of editing? It means I'm not getting rid of all the "so" and "just" in my current manuscript. Of course, I have to be careful of them, like any writer would, but my main character S, who is also the narrator of the story, is going to say things like "so, like, what are we talking about here?" Not every time she opens her mouth, but certainly more than I would write it if I were conforming to some ideal of "good writing."
This is what editors mean by "voice," I think. A manuscript that is edited enough to be readable and interesting, but not soooo edited that it sounds generic and boring. It's a fine line, and one that changes from story to story, character to character.