All you need to do is get their attention. They are there to make money, if you can in any way get them to notice you, like walk by them, stop, look directly at them and nod a hello, they will get the message. Or, if they're with a bunch of other girls (or guys for that matter) go right up to them, get close, get their attention and quietly say "when you get a minute, I'd like to see you, I'm sitting over.......wherever". She knows you're interested, will file you in the back of her mind and hook up with you as soon as she can. Remember, they are there to make money and yours is as good as anybody elses.