you wont appear creepy if you just relax your facial muscles at first, and just put on a little smile. try practicing in a mirror. try and manipulate you facial expression so that you put on the right one when required.
there is a technique when gaming a girl that you can subliminally increase the sexual tension between you two. imagaine there is an up side down triange on the girl's face. each corner for the eyes and one for the mouth. when listening to the girl, alternate your focus starting from the right eye, then the left eye, then look at the mouth, then relax, and then do the cycle again (look at each part for 1.5 seconds max).
hope that i have helped. happy gaming dude
