Erm, what I did was to befriend her first and play the angel's advocate. Explain to her that you like someone else, and make sure she knows the other person to lower her defenses of u.
Now to the basics, u work with her, its easy to show ur value cos she knows u like someone else, what you do is to demonstrate and verbalise how much u care and like the other girl, like future dreams, goals and plans etc.. and get her to tell u stuff about her bf.
There will be some cracks in her relationships, ur job is to amplify the cracks and make her see that you can fill it. When she is feeling down about her current boyfriend, use reverse psychology, you have to encourage her to GO BACK to the boyfriend by sa...
*looks around*
I can type more, but I think the mass of the DJs will kill me if i continue to type.. so... stay cool, there are a million piles of women out there, go grab one in the midst of those millions.
Peace out.
PS: out of those millions, about 80% of them are already attached. =_=