How can I help a friend with low self-esteem when I've tried everything?

My friend is a 17 year old girl who is pretty, smart, a little nerdy (but who isn't?) and is completely dissatisfied with herself. She'll start crying in front of our group of friends because she'll feel "ugly and fat," weighing a healthy 114 at 5'4 and constantly cries herself to sleep as well as shut out her friends because she feels we are "skinnier and prettier" than her and she hasn't been asked out (by teenage boys with raging hormones I might add). I've talked to her too many times, in many ways, of which nothing has worked. A group of us have even gotten a hefty list of signatures of people who think she's beautiful, still with no avail. What can I do when talking and most everything else is out of the question?