First of all, don't get to drunk at the bar if you want to talk to any girl. This will help you determine if she needs "rescuing". She could be bored as hell talking to the doofus, so you have to watch her body language. If she's not looking at the guy, smiling, and she just has a bored gaze on her face and she's looking around, jump in there and rescue her!! Especially if she sees you seeing her, and she has that "ughhh, get me out of this!!" look, then go up to her and say some shit like, "I've been looking all over for you!! *so and so* just got here, come say hi!!"
This move is risky, to say the least(you have to be 100% sure she is bored out of her mind w/ doofus, you'll gain experience by just observing how she acts), but she will be so impressed you took the initiative, she's probably already thinking about after hours with you.
Obviously, you have to take into consideration she could have a b/f, so don't push anything; just say "you looked bored as hell back there, it was painful to watch!" or "it looked like you needed rescuing!" and start bs'ing and see how she responds.