Great advice from Roald Dahl (via Lifehacker) on keeping your momentum going on big projects: leave the last task you're working on before putting the project away unfinished. I always do this when working on long writing projects, like novels: I stop mid-sentence at the end of each session. That way, the next time I sit down to work, I can type several words without having to be "creative," and by the time I've done that, I'm back in the groove.
"When you are going good, stop writing." And that means that if everything's going well and you know exactly where the end of the chapter's going to go and you know just what the people are going to do, you don't go on writing and writing until you come to the end of it, because when you do, then you say, well, where am I going to go next?