The Devil, or evil spirits, will be lurking behind you on the left side. Salt is sacred and wards off evil. If you spill salt, you've desecrated something holy, and you must pick it up with your 'good' hand (right) and throw it over your left shoulder to blind the Devil/evil spirits so that they cannot take the opportunity to do you harm.
Also salt was seen as 'pure' simply becuase it was the white-est stuff around. Oh and they used to throw salt (not rice) at weddings.