Maybe it's just me, but I don't think you have much to worry about.
90% of the people at the clubs/highschool dances can't do much more than sway back and force to the rhythm and maybe do some random arm movements anyways. Some can't even maintain the rhythm. I've always been curious if they get bored of doing the same thing for an hour or two straight though. Almost feel like interrupting their dance and saying "salsa buddy", then walking away.
If you just want to blend in (please aspire to more), just follow the rhythm with your knees... double up on each side before shifting your weight if you have some fast music (typical club music). You might run into trouble if you dance with someone (not partnered, but just with them) for an extended period of time. It'll probably feel awkward doing the same thing over and over again ad nauseum, so just toss in some random arm movement or something so it doesn't seem totally bland.
Anyways, try practicing at home. Get some music blaring, close your eyes and once you get to the point where the music consumes you, it won't feel awkward or embarassing anymore.
