How Do I Get Revenge On a Backstabber?

My "bestfriend" is a little slut..and i know she is, but shes always respected the guys i liked. but THEN at the dance she was grinding hardcore on the dude she knows i liked..and they bugged me a little..but then she told my other friend that I have no chance..and I'm not gonna make a move, so why should that hold her back? she already tried to get him before..and he rejected her..she gave up, until i started liking him and we've been getting closer than she and him ever were. she just started to flirt with him again, with more intentions then just making a good friend. and she is about to be in another relationship..ha. sooo i just want some playful revenge. :) this is the second time this has happened too..and im feeling like an idiot being such a good friend and not getting even a little mad at her.