My own MO is based on advice given by Harlan Ellison:
Commit to at least 1000 words a night. If I can maintain a quota, then I can feel better. And since I'm usually working on 7 or 8 different things, I'll try and get 1000 words down on 3 or 4 of them.
Never finish the scene, unless you can finish the story. If I write "And he went to kill her, and he said to her --" I'll stop there, so I can pick up the momentum when I come back to it.
I'll always stop when I'm hot. Never cold, because it plays into the momentum from the last point. If you can see the logical conclusion of where you're going, it makes it that much easier to pick up and go.
This is great advice. When working on novels I try to stick to the goal of a certain number of words (usually using the NaNoWriMo model.)