Sfalexi said it so true...

The worse they can say is no, but you should have the mind set when you approach her as if she's going to say yes =)
I guess mine is like maybe 10%? one or two out of 10? Most of my friends is about there, some more, some less. Don't forget too, it depends on the place, atmosphere too... I.e. I haven't had much success in picking up a girl at an Emergency Room >.<