I used to be good at dancing in High school-- I haven't done it in practically forever. All you need to do it is some good music and some balls.
The key is to move your hips constantly. Freeform dancing is like vertical sex, in a way. Just loosen up, add some variety to your moves, and then go out there and have fun with it. Feel the beat of the music and move your hips with it, in circles, back and forth, etc at random. Just get into it and feel confident in your abilities, and the rest will handle itself.
Even if oyu know zero technique and just go out there and dance like an idiot, you will get good results if you do it with absolute confidence.
If you do it right, you will have at least 3 chicks grinding you simultaneously on the dance floor (I'm not here to brag, but that's what happened when I first learned how to do it in junior year-- by senior year I had become somewhat of a dance legend)
My signature move is to dance with a chick in front of me during the fast songs, and when the music picked up, I would hold her hands and lift them up high. THEY LOVE IT!
Also, be prepared to sweat. A lot. Those dance floors get HOT, so keep yourself hydrated and stop for a break every so often. Also, double up on the deodorant before the dance.