I find that when I'm on top, if I sit straight up it is very difficult for us to have the in/out thrusting; instead, it's more of a grinding that we both find very sexy. In order to have the in/out thrusting, I lean forward and brace myself on the headboard or wrap my arms under his neck, and he grabs onto my hips and we both work at the thrusting. I find this kind to be very pleasurable. :)