A little elaborate, but bear with me.
So this has to be done at a bar that uses ice cubes in their drinks.
Take up a seat next to the prettiest girl there. She'll probably be wondering why you sat so close with so many empty seats. This is good---pique her interest, get her thinking about you.
You order a gin soda or any other hiball (preferably something with no sugary soda). Sit there and drink quietly while you glance up at her every once in a while with maybe a smile or two. Again, keep her thinking "who is this guy and what does he want?" This is critical.
Do this maybe four or five times and when she finally turns to you to ask "can I help you?" you take a piece of ice out of your glass, place it on the bar, look her dead in the eye and smash it with your cup. Then you say, in your most James Dean with Wolverine claws voice: "Now that I've broken the ice, can I buy you a drink?"
Boom. Panties, meet floor.