One man in a pub/club I go to came up to me and said in my ear, "I'm sorry, even though I'm gay, I just had to tell you you're *****ing stunning darling"... and he was sober as well! It was weird but so sweet of him lol.
My boyfriend (well, before he was my boyfriend and just a friend of a friends boyfriend lol), came up to me in the same pub lol, and said, "Do you know you're really beautiful? I'm not gonna bother using a chat up line, that's not me, but you are seriously gorgeous and I hope you have a great night, see you later maybe?"... obviously it worked, cos he's my boyfriend now!
Just a nice compliment, on looks or something else, but don't make it sound as though you're trying to chat a girl up, be as though you're speaking to her because you want to get to know her as a person, don't use cheesy lines and don't be pushy, and don't constantly offer to buy her a drink, that gets annoying.
x