A word or sentence must EARN the right to live.
I love the wording of that. A sentence or word must work hard, must prove itself on the page - must prove that it works, that it fits - before it earns its right to be there.
I'm in the editing process right now, and I have something to add to that great bit of wisdom. It's more of a reminder to myself, that this is a priority:
A storyline, character, or scene must EARN the right to live.
When nip/tucking your novel with a sharp, careful eye, I think it's important to ask yourself constantly - "Does this character/scene have a reason to be there?" In fact, sometimes I go so far as to pretend I've cut it, then ask myself, "Did that cutting change, affect, or hurt the overall story?" If the answer is "No," then it should probably be cut out.
Now, of course, it's never that simple. There are sometimes-delicate or complex plots where certain extraneous parts should be eliminated, and certain crucial parts must stay. And difficult choices must be made. That's where things can get sticky or stressful.
In the end, I try to use my gut. To do what's best, overall, for the book. Not what's best for me, the writer (sure, it's difficult sometimes, wiping out whole scenes that I'd worked very hard on, or changing plotlines in order to serve a better story purpose). But my goal, my end game, should be to build a better, stronger book.