According to superstition, you should throw spilled salt over your left shoulder to ward off the Devil. Spilled salt is a symbol of bad luck, and the tradition holds that the Devil waits to pounce on anyone so unlucky. Tossing the salt in his face will ward him off.
Salt has been considered lucky or protective for thousands of years by many different belief systems. In the painting "The Last Supper" by Leonardo Da Vinci, Judas Iscariot knocks over the salt with his elbow, portending the darkness that will eventually befall him.
Salt is thrown over the left shoulder because the left side is traditionally considered sinister or unlucky compared to the right.