You can stop biting your tongue now
Joseph Stowell's compelling but brief treatise on "curbing the tongue" will speak deeply to your heart to restrain and sweeten your speech better than any motivation you have ever had.
Reminiscent of the style of Jonathan Edwards, but in simple, modern English, Dr. Stowell systematically categorizes and distinguishes all the ways we can offend or honor God and our neighbor with our speech. He offers insight and clear illustrations of our less-than-honorable tendencies.
More importantly, he uses scripture to reveal God's heart in the matter in detail, and in doing so, provides the best incentive for leaving bad habits of speech behind.