People who talk while a musician is performing drives me right up the wall, unfortunately shhhing them adds further to the noise, so my sister (a musician herself) points to one of the talkers and very politely asks if the person is a musician, generally the answer is no, she then asks if they work for a living, the answer is generally yes. My sister replies, "well I'm a musician, I'm working, so will you please let me do my job" The approach is quite effective. A slight thread drift here, rudeness on forums is something else that drives me crazy.