Well I work in retail, which is one of the worst jobs to have during the holidays. When people ask me how I'm doing I lie and say I'm doing wonderfully or fabulously or fantastically and over the course of the day it actually helps improve my mood or keep my mood up depending on the day. It has to do with that whole idea of the power of words and how they can effect people. Your own words effect you too.
Another thing I do is make sure I have treats for myself on my break or I splurge on a Starbucks when I've had a really bad day.
I have coworkers who go somewhere the customers can't hear them and scream for a while when they have a really bad customer.
We also try and keep our moods up by bringing in treats for the group as a whole. It's a great feeling to start off your day with a cookie that someone made to make you happy. :)