I've worked fast food registars and have done telephone surveys, and I've adopted one mentality while on the job that helps immensly. Just tell yourself these people dont' know any better. Perhaps their parents didn't teach them proper manners, maybe they never went to school, or maybe they really are retarded. Regardless, it's not really their fault, so it's hard to get mad at them.

True, off the clock I fully realize this isn't the case, but a good amount of self delusion goes a long way.