I was just reminded of a great quote: "Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away."
This is by Antoine de saint-Exupery, writer of the favorite little book, "The Little Prince," among other things.
I think this is the quote I would try to teach most if I taught writing. In the end, it's easy to add more and more to a novel. The writer's mind is always coming up with more stuff. But when you're editing, and rewriting, the key comes to cutting stuff that's superfluous, and this is hard. As Antoine says, the moment you achieve perfection is when you can no longer take anything out no matter what it is.
Obviously, not all writers and all books need to follow this example. Bloated books like Stephen King's "It" and Melville's "Moby Dick" are among my favorite novels. But for myself, this mantra, of cutting, helps me realize what I want to realize without losing my goddamned mind.