you tell her the truth, and you don't give her mixed messages. i'm betting here that you have already hooked up with this girl and now you are trying to brush her off. if that is the case, that means that you do not repeat the behavior. you will look like a player and a jerk, but it's better to be seen as a short term jerk than a long term jerk, when you string her along. friends with benefits doesn't work in the long run.