Now that I've shifted to short stories for a while, one of my new "rules" is that if I cannot get a clear idea of where a story is going in a week, I move to another story. While I am a strong believer in "finish what you start," I think you also have to know if an idea is not working. If I need more than a week to finish a short story, that's fine, but if I don't even know where it's going after a week, then it is best to try something else.