Keep the door of my lips.

"No prayer should be oftener spoken by us than that of David in one of his psalms: "Set a watch, O Lord, before my mouth; keep the door of my lips." There is nothing in all our life to which most of us give less heed than to our words. We let them fly from our lips as the leaves fly from the trees when the autumn winds blow. Many people seem to think that words scarcely have a moral character. They watch their acts, their conduct, and then give full license to their tongues. This is not right. A true Christian should have a Christian tongue. Words have terrific power for harm if they are wrong words, and blessed, immortal power for good if they are holy words. We need to pray continually that God would keep the door of our lips and set a watch before our mouth. Only love should be permitted to interpret itself in speech. Bitterness and all evil should be restrained." 
  "In Green Pastures" for December 16th by J.R. Miller, D.D.


Plus some Scripture on the subject. (NKJV)

"Let no corrupt word proceed out of your mouth, but what is good for necessary edification, that it may impart grace to the hearers."
 Ephesians 4:29

"Let your speech always be with grace, seasoned with salt, that you may know how you ought to answer each one." Colossians 4:6

"...sound speech that cannot be condemned..." Titus 2:8