[Tall One In Black]

I think it's always best to ask for help if you're being molested in a club. If you fight the guy off, he's likely to just go harass some other woman. But having a bouncer throw him out protects you and all the other women (and possibly men) in the club.

EPS has good suggestions. Stepping on the foot is particularly good because you can do that even if he has your arms pinned. Failing that, I'd probably go for an elbow to the throat or finger to the eye.

Any counterattack that's ineffective has the potential to make an attacker angrier, but hitting a guy in the balls probably has the highest chance of pissing him off and causing him to come at you more violently.

I'm fortunate that I've never been molested in a club. Probably that has something to do with my size and a lot to do with my "dancing" style - jumping, stomping and generally flailing about so that anyone within three feet would likely be knocked unconscious by a swinging appendage. :P