The reason you throw salt over your shoulder, it is believed that the devil is sitting on your left shoulder, I presume because you spilt the salt. Not sure why this should be, but that's why you THROW the salt. :-)
The myth is that the devil is always waiting for a chance to make mischief. He nudges you, mking you spill the salt, but you throw a pinch over your left shoulder which should hit him in the eyes, and ensure he looks elsewhere to do his wrongdoings.
As salt was an expensive commodity it was a 'sin' to waste it. So if you spilt some and the devil was believed to sit on your left shoulder, you threw salt at him to blind him, so he could not see your sin and could not use it against you on the day of reckoning