Try this little exercise: at the beginning of the day, say: “I won’t speak without thinking of what I say.” You believe, don’t you, that you think all that you say! It is not at all true, you will see that so many times the word you do not want to say is ready to come out, and that you are compelled to make a conscious effort to stop it from coming out.
Ref: Questions and Answers 1950 – 1951