Chick-fil-a figured this out years ago. Their employees are required to be nice and kind, and they can get fired for having a crappy attitude. This sounds awful, Stepford-wifeish, but in reality it improves the working environment for the employees and for the customers, and it costs practically nothing. The stores aren't wildly successful simply because their (let's be honest) sorta bland chicken sandwiches are indescribably delicious. The last interaction you have with the store is a pleasant one, and that's why you're happy to go back.
(Raising Cane's does this as well, and their chicken is better.)
Sending out thank-you notes is relatively cheap, and it has an impact.