Some famous writer--I forget whom--said that he would quit writing when he was on a roll. When he was trying to write and having trouble, he would stick with it, because then when he started again he would be just as stuck. Instead, he would stick with it until he managed to work through whatever and then when finally things got going, he would stop.
A woman I went to graduate school with had a sign on her bulletin board that said "PARK ON A DOWNHILL." This apparently is a key piece of advice from How To Write Your Dissertation In Fifteen Minutes A Day. The idea is that you should make sure to end work with a clearly defined idea of what you are going to do when you start work again. That way, when you start work, you don't sit there and try to figure out what you are supposed to be doing, which can take hours or, for more than one person I went to graduate school with, the better part of your twenties. I have been trying to do more parking on a downhill lately. I feel instead like someone who stumbles hungover and hopelessly-late into his driveway in the morning only to be reminded that he had boozily smashed his car into the garage door the night before.