Honestly, When it comes to the Bar/Lounge/Club scene, the method/style to approach is how attractive the girl finds you. If she's very attracted to you and checking you out a lot, you can go in missile hard and waste no time. Go up to her and stick your hand out for a handshake, once she takes hold of your hand, Move in close to her ear and say "Hey, I saw you walk by earlier and thought you were gorgeous". This line will work greatly again if she thinks your hot because even though that line is B.S that you saw her "earlier" tells her in her mind "this hot guy was checking me out hehe".
Then proceed with, whats your name, where are you from etc.. take it from there. if you see her drink is low, offer her a drink.... take hold of her hand as she follows you to the bar or if your already close to the bar, slightly put your hand on her waist and again move in towards her ear and say "what do you want?". Get the drink for her and start some small talk then either head to dance with her, If she doesnt want to dance, dont take it personally because it may not be you, it may be her feeling embarrassed because she feels she might look stupid. Thats Ok, Keep chatting or take her to a couch etc. Play it from there.