I am not the most experienced on this site, but today in fact I neg hit girl just ever so slightly in a playful kind of way and she laughed and she opened up a bit right after that.
In fact my friend was asking this girl that worked at the place a question and her reply was kind of wrong and he knew the answer, and I was like dang he is doing your job better than you are. Seemed to work wonders, but I had already been talking to her for a couple of minutes so it wasnt too awkward, and I did it smiling, so in certain situations it worked wonders, but its better to do on intelligent girls because then they dont get all offended as easily.