This is a general question that gets asked a lot. The actual answer doesn't depend on where you are (in this case a bar), it depends on who you are and where you are in your life.
Actually starting decent conversations happens long before you actually walk into a bar. Did you dress decent? Check your stank, do you smell good? Hows your comfort level in your own skin?
A lot of people think their problem is what they are saying, when in reality its what they are doing. Most of the best sets you'll open wont require any great opening line, you'll be suprised at how little that matters when you're constantly dealing with complicated relationships.
Anyways, I usually feel like I look pretty decent when I go out. I had caught the last part of 8 Mile on some channel and saw how Eminem had reversed everything on Papa Doc at the end of the movie. I thought it would be funny to try "Opposites Opening" for a night and its actually kinda stuck with me.
It fits me because Im a decent looking guy (not a model or anything spectacular, but I clean up nice) and me AND the girl Im opening know that Im bull****ting. She knows Im flirting. That makes things easier.
You can give it a shot, see how it works for you.
"Hey, hate to bother you, Im really only here for the big girls. You're teeth are way to white for me, but I guess I can stand you for a few minutes if you're into guys with really small hands."
She usually checks out my hands, grabs them, whatever. Tells me they arent small, I tell her they are in the hands of the women I like. Its funny. She laughs... carry on.
You'll quit worrying about opening when you start having fun with it and try things that you never thought would work. Thats how you'll really find what works for you.