I don't know about always stopping when I'm going good, but I almost always leave myself with a concrete "next step" for the next day when I do decide to stop.

I don't know about always stopping when I'm going good, but I almost always leave myself with a concrete "next step" for the next day when I do decide to stop. Sometimes this means getting the task I'm working on to 95% completion, or forcing myself to work a bit longer so that one of tomorrow's tasks is already started and approximately roughed out (tests written or methods stubbed or HTML roughly laid out). If the day's boundary falls on a major task boundary, the next day starts with me figuring out what to work on next - not nearly as conducive to "getting into the groove" as tying up a few loose ends and then jumping on whatever I need to work on next.