Basically, superstitions are either signs which people observe to gain good luck or to stay clear of bad luck. People who take superstitions very seriously are often inspired by a belief in the supernatural. There are still others who believe in superstitions however much they are disproved.
Superstitions have evolved over a very long time. People just seem to need something, or someone, to blame if their life goes drastically wrong. For example, if someone has some unexpected bad news it can be more acceptable for them to put the blame on the ladder they walked under earlier that morning, rather than face a difficulty as their own responsibility.
There appear to be more superstitions about avoiding bad luck than there are about creating good luck. Perhaps this is because people like to think that if something good happens it is due to their own hard work or because they deserve it, whereas when something bad happens people don't like to blame themselves.
Below are some of the more common superstitions:
Good Luck Superstitions
Horseshoes and lucky charms such as stones with natural holes through them bring good luck.
Rubbing rabbit feet. The rabbit is obviously not a lucky one.
A black cat crossing your path is good so long as you don't run it over, otherwise it might be seen as a rather unlucky omen - especially if you happen to meet its owner.
Throwing salt over your left shoulder after spilling some. Apparently a devil is sitting on your left shoulder, and the force of the tiny salt particles knocks him off and he lands in a little heap on the floor. Then you should stamp on him.
Finding a four leaf clover.
Finding a pin on the floor and picking it up. Presumably this is only lucky if you see it before you tread on it.
Bad Luck Superstitions
Breaking a mirror leaves you with seven years' bad luck.
Putting shoes on tables.
Opening umbrellas indoors.
Seeing a single magpie.
Walking under ladders.
Wearing amber that someone else has previously owned.
Throwing salt over your right shoulder as it knocks off the angel who sits there to balance out the devil on your left shoulder.
When a bride and groom see each other before the ceremony on the day of their wedding.
Stepping on a crack in the pavement. ('Step on a crick and you'll marry a brick and a spider will come to your wedding.' Now where did that come from?)