In ancient times salt was an expensive and valuable commodity. It was a symbol of trust and friendship. Spilling salt, therefore, was considered wasteful and distrustful. It is believed that this is where it became "Bad Luck" to spill salt.
The superstition of spilling salt has a second part. You take a pinch of the spilled salt and throw it over your left shoulder. The purpose was to throw salt into the devil's face and stop misfortune from happening.
Interesting Fact: Leonardo da Vinci's 'The Last Supper' shows the betrayer Judas Iscariot having knocked over the salt-celler.