One thing that separates the professional artist from his amateur fellows is knowing when the piece they're working on is finished. The amateur often holds a mistaken belief that more time spent on a painting equals a better painting.
Nothing could be father from the truth. In actual fact even the professional artist can work on a piece past the point where it was well done.
My advice: "Let it Be" three simple words that can save you. While working I feel it's best to try and accomplish whatever is happening on the canvas/board in as few steps as possible. Just tell your self to let the art be. Let it breathe.
"When do I stop though" you say?
There is a still, quiet voice that tries to tell you when to stop. I call it intuition. We are going to talk a lot more about intuition later but for now I'll say, "intuition can guide you step by step if you listen to it".
Like any other skill it must be learned though. As you practice listening and acting in accordance with your intuition it becomes stronger and stronger.
Be aware that it is very easy to drown out this subtle guidance. There's nothing forceful about how intuition communicates. Though it speaks in a whisper it has the power of a hurricane.