I have the exact same problem, you can use this fact to your advantage, whenever I'm just standing around idly, my face is very neutral and I actually a little pissed off.
People always ask me what's wrong with me when I'm not interacting, anyways, this forces me to get into set and engage people, then I'll start making different facial expressions.
Plus, you're out in a night club, force a smile if you have to, I hate forcing it, I'd rather just feel organic about it but that means I need to be in set.
Always be in set, that will help your game and alleviate this problem.
-Kevin