Imagine a job interview. There is a big difference between the person who immediately rolls over on his back and begs for a job and the person who knows they have got as much to offer the company as the company has to offer him. When you're with a girl, don't act like you're interviewing to be her partner. Imagine instead that you're interviewing her to be your girlfriend. Putting yourself in the power position will not only make you more attractive to the girl, but it will keep you from choosing the wrong girl just because she'll get with you.