Remember that customers are not problems. They are your business.
While on road trips I try to eat reasonably healthy. I will regularly stop at a grocery store for a healthy alternative to the regular deep fried fair of the traveler.
On a recent stop at a grocery store I was getting some blueberries to accompany my dinner choice. When the cashier tried to ring in the order the berries would not scan into the system. She was annoyed. Muttering, she walked over to the phone and rang for assistance. Meanwhile I was left standing for several minutes waiting.
Understanding that this was not her fault, and that she was annoyed, I tried to lighten the mood by making small talk. No answer.
Once the manager came to perform a manual override I paid the clerk and said thank you. No answer.
This clerk made the number one client service mistake. She transferred her annoyance of the system onto the customer. It became my fault. All I wanted was some blueberries.
The most important thing that any customer facing employee can do is remember that customers are not problems. They are your business. Whether you are having problems with a cash register or your customer is being difficult on a contract, or whatever the issue is, they are not the problem.