What? You don't have the balls to just tell it like it is and break it off with her? Here's a tip:
Act like an ass and give her friends plenty to hate about you.
Her friends will start doing all your work, convincing her to dump your sorry ass.
Of course, the converse is also true. Nothing gets your way to a girl's heart faster, then your ability to make her friends love you. If you can inspire a little bit of envy, watch out.