Spilling salt is bad luck because in Roman times, salt was so valuable that soldiers were paid in salt rather than money. Spilling it was equal to burning money. I'm not sure where throwing a pinch of salt over your left shoulder negates the action of spilling, but I can tell you that it is where the word 'salary' came from.
盐洒出来是不吉利的象征。因为在古罗马时代盐是很珍贵的，士兵们的薪酬领的是盐而不是钱。洒出盐意味着烧钱。我不确定是否可以通过在左肩上撒点盐来消除背运或盐洒出的影响，但是我可以告诉你“薪水”（salary）一词来源于盐(salt)。