What you need to do is be there for her and make her feel special. Compliment her, tell her that she is beautiful and pick out certain things about her, the things that you love about the way she looks. If she does something different with her make up, her hair or has something new on make sure that you comment on it and just generally try and make her feel good about herself. It will be really difficult for you to get her to listen but when she does listen, she will feel better about herself. Don't go over the top with compliments though because it might seem fake. Just be honest with her, tell her how you feel about her and her looks. She will listen to you eventually.
Edit: After reading your last post, I don't really know. I used to feel the same way that she did and I also have a history of self harm. Whenever I feel like that my boyfriend tells me how beautiful I am, he compliments me on things and just makes me feel really loved. I will ignore it at first but in time I will listen and I do feel better about myself. It's hard to get someone to listen to you when they are feeling like that but give it time.