While I completely understand that customer service is a part of the job when someone works as a cashier, waitress, etc, I think people who haven't worked in that industry (or in a very long time!) don't understand how truly E-V-I-L some people can be. People seem to think it's okay to treat people in customer service roles as subhuman verbal punching bags, and yet we expect these people to take the verbal abuse with a smile and snap right back without letting it affect them. Most of the time, the "problem" genuinely isn't the fault of the person you're speaking with - it's the chef, the backroom, the warehouse or simply store policy.
I think compassion, kindness and understanding go a long way. People seem to forget that with every interpersonal interaction you're responsible for 50% of what happens. 50% of any good or bad experience is YOURS, and you need to own that. Most people just blame the customer service rep - it's easier.