Be the better person and walk away. I think you just become more desensitized to it as you get older. Don't let it bother you, or at least try not to! Chances are, if someone is talking about you, most people don't see that person as really nice. They probably don't believe that person either. When I met my BF, his friends were very nice, but very critical of him. I wondered why they said some of the things they said about him, but I realized that they just did not know him. There is no way they could think such things about such a great person.
Ask them in front of people why they talked behind your back and then before they answer or as they start, say...........forget it, i have no room in my life for backstabbers, see you around..................and walk away