Cross your fingers
Those wishing for luck will often cross one finger over another, a gesture that's said to date back to early Christianity. The story goes that two people used to cross index fingers when making a wish, a symbol of support from a friend to the person making the wish. (Anything associated with the shape of the Christian cross was thought to be good luck.) The tradition gradually became something people could do on their own; these days, just saying "fingers crossed" is enough to get the message, well, across.
No umbrellas inside
… And not just because you'll poke someone's eye out. Opening an umbrella indoors is supposed to bring bad luck, though the origins of this belief are murky. Legends abound, from a story of an ancient Roman woman who happened to have opened her umbrella moments before her house collapsed, to the tale of a British prince who accepted two umbrellas from a visiting king and died within months. Like the "don't walk under a ladder" superstition, this seems to be a case of a myth arising to keep people from doing something that is slightly dangerous in the first place.
Friday the 13th
If you're not scared of Friday the 13th, you should be scared of the word used to describe those who are: friggatriskaidekaphobics. (An alternative, though just as tongue-twisty, word for the fear is "paraskevidekatriaphobia.")
For a superstition, the fear of Friday the 13th seems fairly new, dating back to the late 1800s. Friday has long been considered an unlucky day (according to Christian tradition, Jesus died on a Friday), and 13 has a long history as an unlucky number.
According to the Stress Management Center and Phobia Institute in North Carolina, about 17 million people fear Friday the 13th. Many may fall prey to the human mind's desire to associate thoughts and symbols with events.
"If anything bad happens to you on Friday the 13th, the two will be forever associated in your mind," said Thomas Gilovich, a psychologist at Cornell University. "All those uneventful days in which the 13th fell on a Friday will be ignored."