One of the tenets of Zazen Meditation is to keep a gentle smile on your face during your practice. They say that Zazen is all about enjoying your existence. To just sit quietly and be joyful of being there.
What my teacher told me is that once you put a smile on your face your entire focus goes on maintaining that smile. All thoughts tend to stop automatically. It happens because we are so used to have a dull impression on our face that it takes immense effort and focus to smile without thinking about it.
I have found this to be absolutely true. It is an amazing little trick, a hack for the monkey mind. Even if you don’t meditate just try it. Do it for 5 minutes and see how quickly you reach the ever smiling, thoughtless state 🙂