Know when to stop working. The creative process has a natural arc to it. Don't push on when inspiration is exhausted.
These are hints that it's time to stop writing: you feel irritable, indecisive, or lost. If you find yourself looking at the same thing for ten minutes without making progress, then it's probably time to quit and plan to come back another day with a fresh perspective.
Of course, putting your work aside isn't always an option. If you face a deadline and must keep going even when you feel creatively exhausted, it's time to lower your standards and find the simplest possible way to get to the finish line. That's when you should fall back on proven formulas, cheap gimmicks, and familiar cliches. Steal from your own previous work if you have to. Turn in something that's workable and usable. But don't struggle to come up with something fresh, new, and brilliant when that creative spark isn't there.