Flattery will get you anywhere:
John Seiter and Eric Dutson of Utah State University, recruited two hairstylists who, after cutting their customer's hair, randomly pulled one of three pennies from their pocket.
If the penny was marked with a 1, they gave the customer no compliment. If it was marked with a 2 they told the customer: "Your hair looks terrific". If it was marked with a 3 they said: "Any hairstyle would look good on you." The amount tipped by each customer was then recorded by the hairstylist.
You guessed it: the more compliments the bigger the tip. Worth thousands in a year of haircuttery.