Ok, here is a quick tip.
Most girls that work at bars are flat out. They haven't got time to stand there and talk to you, because there are heaps of other people waiting for drinks. Sure you could go early.. or really late, when it isn't that busy.. but here is a simple trick that I wish I thought of earlier.

Order something that takes a while to make.
That's it. If you order normal tap beer or a mixed drink and they are done in a minute and you haven't had a chance to chat.
But order something like guiness (where they wait for it to settle) or a ****tail (that they put lots of stuff in), and you have a good few mins to chat while she's making your drink. Do that a few times through the night and you've got a massive advantage over some schmo who tries to hold her up after she's poured his pint and ready to move to the next person.

Unless you have instant chemisty with a bar girl, my advice is to become a regular at the venue that has one (or more) bar girls you like, and build rapport over time. And being friendly with all of the bar staff (male and female) is a must. Chatting up the owner won't hurt, either. In short, make friends with the staff, e.g. make small talk without being a nuisance, and offer to buy them drinks, and then see what happens when you start to game your target. A good challenge, and it passes the time inbetween chasing cute patrons.