Breaking a mirror
One of the longer sentences dished out by Lady Luck, the superstition follows that breaking a mirror will leave you doomed for seven years. The reflection is said to represent the soul, so damaging a mirror corrupts the soul of the one that broke it.
Some believe that the reason for the length of the punishment is that the Romans (the first glass mirror-makers) thought life renewed itself every seven years, so the soul wouldn't be fully restored until the next cycle had passed.
If you have recently broken a mirror, fear not as there are apparently great lengths you can go to in order to save the best part of your next decade:
Throw salt over your shoulder
Grind the broken pieces into dust
Bury the pieces under a tree during a full moon
Place the broken pieces in a river running south
Touch the broken piece against a gravestone