When you are practicing … do not try to stop your thinking. Let it stop by itself. If something comes into your mind, let it come in, and let it go out. It will not stay long. When you try to stop your thinking, it means you are bothered by it. Do not be bothered by anything. It appears as if something comes from outside your mind, but actually it is only the waves of your mind, and if you are not bothered by the waves, gradually they will become calmer and calmer. […] Nothing outside yourself can cause any trouble. You yourself make the waves in your mind. If you leave your mind as it is, it will become calm.
Shunryu Suzuki, Zen Mind, Beginner’s Mind
Mindfulbalance 
I would add:
You will notice over and over that you have been caught up in following a thought and begun an internal chatter. Simply let it go and return to calm watching the waves go by, unmoved by the tempting emotions and opinions.