Standing on a midtown corner late one night, I was almost bowled over by this guy jumping out of the back of the pickup truck. He grabbed the water gun and quickly blasted the garbage can from all angles, including the inside. In about a minute, he had completely washed the can and jumped back in the pickup bed. The driver accelerated to the next can, slammed on the breaks and the process started all over again, leaving shiny garbage cans behind.
I had noticed they always tended to look clean, and now I know how the magic happens.