Once upon a time, I read somewhere (thought I can't for the life of me remember where at the moment) that you should stop writing while you still have something to write. At the time, my thought was, "What?!? That's insane!" When I get writing, I want to keep writing and writing until I run out of things to say and have no more words to put on the page. I keep going and going until I'm totally worn out and have no clue where I'm headed next.
Cue moment of brilliance:
Yesterday, I was forced to stop writing because it was finally time to leave work. I had my swim fitness class at the gym afterward, and I was a little worried that I would forget everything I had in mind for the scene I was working on. However, yesterday's workout consisted of lots of distance sets. For anyone who hasn't done a nice, long distance swim, once you get into the rhythm it's hypnotic. I often use distance sets to plot out what's going to happen next or where the characters are going. It wasn't until one of the last sets, a 400 meter one (that's a long distance in the pool, trust me) that it hit me; having that little bit left to write gave me a building block for the next scene. Instead of having a dead end at which to start, I had a bit of a lead into it. It was fantastic! I think I got more done in the eight minutes it took me to finish the set than I had in hours before when starting from scratch. Brilliant!
So there ya go. Stop writing before you run out of things to write.