Acknowledge people who serve you. Make sure you tell people how much you appreciate that they are there to help you find something, answer a question you may have, give you directions, ring up your groceries, or deal with you over the phone.
Make sure your response has nothing to do with the quality of service they have provided. Even if they have not done a particularly good job, acknowledge them anyway. Even the surliest attendant at the tollbooth gets a "Thank you, have a nice day." No act of service is too small to acknowledge. The elevator operator gets acknowledged. The person who waits on you, takes your coat, asks you to fill out an application, hands you your dry cleaning, anyone who makes it possible for you to get through your day receives an acknowledgment.
And if someone's service is outstanding, spare no praise. Here's your opportunity to let him really know how much you appreciate what he's done for you — even if you're paying for it.— Alan Epstein in How to Be Happier Day by Day