History and psychology of tipping. One interesting set of facts:

In one study, a waitress received fifty per cent more in tips when she introduced herself by name than when she didn't. In another, waiters sharply increased their tips by giving each member of a dining party a piece of candy and then, seemingly spontaneously, offering each person a second piece, too. Squatting by the table instead of standing, writing "Thank you" on the back of checks, and touching customers on their shoulders all measurably improved tips. And waitresses at an upscale restaurant who simply put flowers in their hair boosted their tips by seventeen per cent.

These tricks may seem cutesy, but they help personalize the relationship between the customer and the server, which tells you something important about the nature of tipping. The practice really belongs to what sociologists call a gift economy rather than to a market one.