Breaking a mirror is another supposed source of bad luck. Well, mirrors used to be rare and expensive, so breaking one meant going without, saving for another, and waiting. This was bad. Also, breaking a mirror usually meant that shards of glass would be scattered about the floor.
Which hand do you use to throw salt over your shoulder?
The most common contemporary belief requires you to toss a pinch of the spilt salt over your left shoulder, into the face of the Devil who lurks there.