Salt is interesting. Salt is considered and has been for a very long time, as being a purifier. It is so pure it repels the presence of evil. That is why it is used to cast circles in various magical traditions. Once you are inside the circle no evil can cross to get at you. It is also sprinkled on thresholds to prevent evil entering. That is why the tradition says that if you sprikle salt on the doorstep no vampires can enter... we're evil right Salt is also used magically to purify tools, such as the athame or wand, crystals and other items which are covered in salt which then absorbs the negativity.
Throwing salt over the shoulder 'into the face of the devil' repels any evil that is standing behind you.
The reason you throw it over your left shoulder is because the left hand path is considered to be the path of darkness. The left (in latin sinistra) path is the path of the moon and, coincidentally the path of the feminine mysteries. In ancient, and not so ancient, times left handed people were considered to be posessed by evil and many (my ex included) were forced by their parents to suppress their left handedness and to learn to use their right hands at a very young age.
Just some useless information for a Saturday morning.