Cultivate the habit of thinking before you speak. Pause, slow down.
Realize that what you say to others must be honest in order for you to be true to yourself. And being true to yourself is the only way to go. You are cheating yourself most of all, by being dishonest.
If you know that what you are about to share with someone is a lie. Do not share it. Stop. Slow down. Pause before you speak or act in an unhonest way.
Keep close tabs on yourself as you go through your days and soon, you'll be in the habit of being honest. Not brutal, but honest in your remarks and your demeanor.
Good Luck. The fact that you care enough to want to change speaks volumns!!