In my previous break up, I told something like this.

"I will tell all your friends whatever you want me to tell them. Otherwise, I will tell them it was due to stiff resistance of your parents that we both decided to part ways. And I truly, sincerely believe that in next five years, you will find a person several times better than me, and you will never regret that the break up happened."

Note that if you really want to hurt someone who loved you, then it is a whole new ball game. If your goal is to tell your friends how many girls you scored, it is different thing. But if you are leaving her, because you truly believed that there was a gaping difference between you two, and that you really want her to end up with a better person (this means wealthier, taller, more handsome), then you have to make sure that the life won't become hell for her.

When I left her, I knew it would hurt her for a few weeks. But I always wished that she would find a tall, fair skinned man, who is a cardiac surgeon, who plays golf and hangs out with great people, who is confident and would love her and whom her parents could embrace proudly. I have stopped tracking her life thereafter, but whenever I thought about her, I sincerely wanted her to be the happiest girl alive wherever she was!