By an old superstition it's considered bad luck to spill salt. If one was to spill it in error they then must throw it over their left shoulder. Salt was considered very pure so throwing it over the left shoulder would temporarily stop the devil, therefore preventing any misfortune.

A few quotes ..."He is not worth his salt, is a common expression. It originated in ancient Greece where salt was traded for slaves."

"The first written reference to salt is found in the Book of Job, recorded about 2,250 BC. There are 31 other references to salt in the Bible, the most familiar probably being the story of Lot’s wife who was turned into a pillar of salt when she disobeyed the angels and looked back at the wicked city of Sodom."

" An old English belief has it that every grain of salt spilled represents future tears. The Germans believe that whoever spills salt arouses enmity, because it is thought to be the direct act of the devil, the peace disturber. The French throw a little spilled salt behind them in order to hit the devil in the eye, to temporarily prevent further mischief. In the United States, some people not only toss a pinch of spilled salt over the left shoulder, but crawl under the table and come out the opposite side."

"Jesus called his disciples "the Salt of the Earth." In Leonardo DaVinci's famous painting, "The Last Supper," Judas Escariot has just spilled a bowl of salt - a portent of evil and bad luck. To this day, the tradition endures that someone who spills salt should throw a pinch over his left shoulder to ward off any devils that may be lurking behind."

"In Buddhist tradition, salt repels evil spirits. That's why it's customary to throw salt over your shoulder before entering your house after a funeral: it scares off any evil spirits that may be clinging to your back."

"Shinto religion also uses salt to purify an area. Before sumo wrestlers enter the ring for a match - which is actually an elaborate Shinto rite - a handful of salt is thrown into the center to drive off malevolent spirits."

"Today, a gift of salt endures in India as a potent symbol of good luck and a reference to Mahatma Gandhi's liberation of India, which included a symbolic walk to the sea to gather tax-free salt for the nation's poor."

Easy to understand why the spilling of Salt was and still is considered to be bad luck...

Besides of the "true-superstition-based" reasons, there is a highly pragmatic reason that originated the myth on bad luck when spilling (wasting) salt, and is ....(believe it or not).....it's cost.

Salt nowadays is a very abundant and cheap resource, but at some point in history was very scarce. Also, in our times, salt is mostly a condiment to give more flavor to food, but when refrigerators didn't exist salting the meat was the only way to preserve it.

During the time salt was scarce and expensive, people was even paid to buy salt. That is why the payment for work is known as "salary" (from the Latin "salarium"). This originated when Roman soldiers were paid this way.

Then, the waste of salt, was considered to bring bad luck.

Note: In most cases you will find pragmatic answers to most superstitions such as the ladder-fear, the mirror-cracking, the indoors-umbrella, the third-cigarrette, etc.

From the website for Morton Salt...

The widespread superstition that spilling salt brings bad luck is believed to have originated with the overturned salt cellar in front of Judas Iscariot at the Last Supper, an incident immortalized in Leonardo Da Vinci’s famous painting

http://www.mortonsalt.com/consumer/about_us/history/

This superstition dates way the heck back to when salt was a mighty precious thing. Salt is necessary to sustain life and back then, you couldn't just run to the quicky mart and pick up a couple of pounds for less than a buck. To spill it would be so clumsy / disrespectful / wasteful that only evil could result. Because salt was so essential to life, it was considered a gift from God / gods / earth (depending upon your choice of religion).

It was also used for medicinal purposes. Throwing spilt salt over the left shoulder is linked to its medicinal use. If it could not be administered, the next best thing was to throw it into the eye of the evil spirits that brought sickness upon us. These spirits were thought to lurk behind your shoulder, waiting for an opportunity to strike.

The gravity of spilt salt is illustrated in The Last Supper by da Vinci. The person who betrayed the Messiah, Judas, has the omen of the overturned salt cellar before him. This is a sign of Satan himself.

Here's some more info:

"The widespread notion that the spilling of salt produces evil consequences is supposed to have originated in the tradition that Judas overturned a salt-cellar at the Paschal Supper, as portrayed in Leonardo da Vinci's painting. But it appears more probable that the belief is due to the sacred character of salt in early times. Any one having the misfortune to spill salt was formerly supposed to incur the anger of all good spirits, and to be rendered susceptible to the malevolent influences of demons. When, in oriental lands, salt was offered to guests as a token of hospitality, it was accounted a misfortune if any particles were scattered while being so presented, and in such cases a quarrel or dispute was anticipated

The Germans have a saying, "Whoever spills salt arouses enmity," and in some places the overthrow of a salt-cellar is thought to be the direct act of the Devil, the peace-disturber. The superstitious Parisian, who may have been the unfortunate cause of such a mishap, is quite ready to adopt this view, and tosses a little of the spilled salt behind him, in order, if possible, to hit the invisible Devil in the eye, which, temporarily at least, prevents him from doing further mischief. This is probably a relic of an ancient idolatrous custom; and salt thus thrown was formerly a kind of sop to Cerberus, an offering to pacify some particular deity. In like manner the natives of Pegu, a province of British Burmah, in the performance of one of their rites in honor of the Devil, are wont to throw food over their left shoulders to conciliate the chief spirit of evil."

Read more about this at:

http://www.sacred-texts.com/etc/mhs/mhs36.htm

I heard somewhere that the salt was like the "sands of time" or something, and that it was kind of bringing death closer somehow? I don't know, I can't remember it too well.

salt over your shoulkder is bad luck becuase it affects the gods