Why is it considered bad luck if you spill salt?

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