Tricky questions.
I just tried to act utterly charming. I would say some important things are;
1) eye-contact, lots of it, try and make your pupils dialated.
2) get them to fuck on about themselves, girls love talking about themselves
3) dont reveal too much about yourself, the more interesting you seem the better... however, mysterious does not equal gloomy, try not to be necessarily cheerful, but happy with the general gist of your life.
4) You are happy with your life, but things could be better, be optimistic, this is the essence of acting successful, and as much as girls don't like to admit it, this is what is attractive to them.
5) Be an alpha male, some think this means going around giving people the evil eye, but that is rather adolescant, act in control of yourself, but not up-tight.
6) Express interest in a very subtle manner, indirectly, it is a fact that if someone thinks you like them it is more probable that they will like you in return. But in this case being a friends GF and all this may not be the best thing to do.
7) Manipulate situations of heightened emotions or atmosphere. If you two somehow end up for example, at a party together and are both taking a break outside or something, sitting at a table have a casual talk, take long gazes into her eyes and whatnot, again, dialated pupils help, pose, whatever it takes to heighten the atmosphere around you two.
This is all i can think of, and it does seem rather calculated and Machiavelian(sp?), but it works, i have used it in practice, and i was in the same situation as you, i stole my current GF from a friend i have had since kindergarden, and i dont regret it for a second.
GL
EDIT: As a note, she left him for me before I never expressed any interest in her, and it seemed at first that i did no wrong, but the reality was i purposely set out to get her. Though i publicly admit what i did to all, and always have.