Often the difference between the friend zone and the "sex zone" is simply this: building sexual tension. Two guys who are equally attractive and have equally good rapport with a girl can wind up getting drastically different results with her based only on one guy's willingness and confidence to turn the interaction sexual (and the other guy's failure to do so).

Turning things sexual doesn't mean being rude, obscene, inappropriate or anything similar. It basically means (1) acting like a sexual partner who is not afraid to be intimately close to her and is not afraid to begin making physical contact (i.e. touching her - slowly to start and then gradually increasing as she gets comfortable).

The second component is turning the conversation sexual. This does not mean making lurid sex jokes. Often it is little more than taking "common, innocent statements that she makes" and putting a "sexual spin" on them. Basically interpreting (or playfully misinterpreting) something innocuous as something sexual. By doing this you create the impression that "she is the sexual aggressor." When done correctly, such behavior significantly spikes sexual tension and makes it much more likely that you will attract her as a sexual partner and not a friend.

If you'd like to see a simple example of "framing her" as the one "hitting on you," check out this video.