Hey Phate
Just got out of this sort of thing myself so I've got a few things to say on what not to do
.You've got to find a balance between being a decent person to her (i.e. you're not just in it for the whole whenever you fancy sex you get it concept) and remaining friends for as long as possible. Its likely that eventually that it will tip you one way or the other eventually where you go from there is up to you.
What I found was cool was keeping it isolated to each others houses, while still eating and doing something social like watching a film before screwing around. It maintains comfort and also helps maintain the secret bit to it, my problem was having her SPAM find out who veered mine off me(still mates just no sex).
Resist the temptation to justify talk to her about what it is your doing, I felt that I had to talk to the girl about the whole fuck buddies thing in order to maintain it. Apparently labelling it makes it more taboo, seriously the day after we established we were "sex friends" she decided she wasn't up for that sort of thing at the moment... despite screwing for the last month. You're probably both aware of what the relationship is. Let her talk about it first and if you like what you hear keep rolling with it.
Hope this helps, have fun
R&B