If your girl is one of those who cant get off unless her clit is stimulated, try missionary style and position your body so your pelvis is slightly above hers but your hips are flat against her. Then you can rub the base of your penis against her clit instead instead of just ramming your dick straight in and out of her vag, which gives her next to no clit stimulation at all. I had one gf would liked me to push it all the way in and then just move my hips around in a circular pattern against her clit and it would drive her absolutely crazy.