I find that there are times when it is best to be an utter asshole, but the best default for me seems to be the ability to say really harsh and shocking things with a gentle smile, so they don't know if I'm joking or not. On the one hand they hear you say something really unexpected and nasty, but on the other hand you're not looking or acting threatening or aggressive, so the words don't match... and they come away puzzled and wary. Kind of a warning shot across the bow, as it were.

Good times...