What the cloud said. I would add with when someone makes a comment about your looks - tell yourself. "They must not like themselves very much." or "I feel sorry for them, they must be very unhappy." At first your mind will tell yourself "this is bull* " etc. But keep at it, eventually you will assimilate these thoughts until it comes up naturally.
As for reacting to those comments. It's up to you how you react. The funny thing is when I was feeling poorly about myself I would get a lot of negative reactions. I was too afraid to say anything or do anything except hide. Now that I feel better about myself, I actually have hardly any negative reactions so I never get a chance to practice what I want to do in the situation!