Starting today, become aware of how you are allowing yourself to think, and whenever you entertain a critical or negative thought, think 'stop' or see a 'stop' road sign in your mind. Then, dealing directly with the subject matter, change the negative focus of your thoughts into a positive one. Make the very best of reality as it is. If the habit of negative thinking is deeply engrained, you will need to do this exercise persistently over time in your mind.

-How To Improve Your Self-Esteem, by Elaine Sheehan