It's been a long while since I was actually employed, but when I did work in retail, I found the despair and frustration was in large part due to my belief that customers (and the human race at large) were even slightly intelligent or good people. However, once I came to the conclusion that customers were cantankerous, underhanded, thieving, mindless sheep, who can't read, can't follow simple instructions, will lie at every opportunity, have no sense of personal space or hygeine, and will even soil themselves without batting an eyelid (wish I was kidding here), I found a sense of peace.
The poison, at least from my perspective, was the pain of trying to hold out faith for fellow man against overwhelming evidence to the contrary. I learned to treat customers as if they were mentally challenged incontinent children and found the result to be quite satisfactory. And to cope with stress, I found a quiet place with no one else in sight where I spent my lunchbreaks enjoying the silence and reading classical literature. I exercised heavily on my days off or bent my back at manual labor around the yard. The sweat and ache is surprisingly cleansing. And, on particularly frustrating weeks, I either bought a bottle of bourbon to enjoy a single good raging drunk night or went down to the firing range and worked on my aim.
Aggression is something you can't just bottle up. You need to find an outlet to stay healthy while mentally distancing yourself from your customers. If you see them as real people, you're just in for a world of hurt. Hope this doesn't sound too cynical.