I was security at a casino in Las Vegas almost 10 years ago, now. I learned the fine art of ruining a drunk's good time and having them still happy enough to thank me as I'm walking them out the doors. It was a great experience dealing with bad customers, at their worst and still having to maintain a positive demeanor and a good customer service attitude. 

What I learned was that the customer was not always right, but that doesn't mean I need to lose control and drop down to their level. Regardless of their behaviour I need to always be a professional. 

The positive side of that, is learning that even in a job that isn't traditionally a role designed to keep people happy, I could make a positive difference. Understanding the customer's needs and adding a touch of humor can really build up a positive relationship. It wasn't the big things, it was the small surprises that helped to improve someones experience.