Colossal Art & Design has a neat pictoral post here about money trees, and notes:
Apparently in several wooded areas around the UK, passersby have been stopping for decades (if not centuries), meticulously hammering small denomination coins intro trees . . . the practice might date back to the early 1700s in Scotland where ill people stuck florins into trees with the idea that the tree would take away their sickness.
I don't think spiking a tree with coins will ward off illness, but I am heavily invested in my own belief that drinking a glass of orange juice every day prevents colds (and since I've only had two in the last five years, there may be something to that.) I have no problem with the number thirteen; it's eight that always makes me a little nervous. Black cats can cross my path any time, but I won't voluntarily walk under a ladder -- and that's just common sense; I've treated too many people who did and got head injuries. Wearing something green seems to help me write better or otherwise brings me a little luck.
Probably my worst superstition is that I flatly refuse to look at anyone when for any reason we physically part ways; three times in the past I've watched someone leaving until they were out of sight and all three died before I could see them again. Do I think me watching them go made them die? Not at all. So why don't I put aside my silly superstition and watch and wave goodbye? I guess it's because I could be wrong.
Bestowing on your characters such habits is one more way of breathing life into them. Traditional superstitions are not your only choices; you can invent a ritual or avoidance behavior tailored to your character's personality, setting and/or backstory. The superstition doesn't have to be logical to anyone but the character, either. A protagonist who grew up desperately poor might keep money in strange places, or compulsively collect piggy banks, or can't pass a homeless shelter without going in to make a donation. A character who nearly died in a bad car crash might hang a good-luck object from their rearview mirror (maybe even something from the car that crashed.)
It's a good idea to research your character's cultural background and learn about their superstitions, as these are often passed down through the generations. Foe example, the numbers four and nine are considered bad luck by the Japanese for these reasons:
The number four is pronounced as “shi” in Japanese, and is the word for death. The number nine is pronounced “ku” and rhymes with “kutsuu” which means pain in Japanese. The number four and two together are pronounced “shi-ni” which means to die and as a result the number forty-two is considered unlucky as is number twenty-four or “ni-shi” meaning double death.*
Now I'm off to finish basting a quilt piece I was working on this morning. If you leave a seam undone for too long your thread will become mysteriously/hopelessly snarled and you'll have to pick it out and sew it over; ask any quilter. This is because you gave the devil ample time to mess with it . . .
Corsinet.com has collected lengthy A-L and M-Z alphabetized lists of superstitions.
The Origins of 7 Common Superstitions by Jill Harness
Urban Legends Online is an excellent resource for all manner of superstitions
*from The Unusual Superstitions of the Japanese by Shane Sakata
Money Trees article link swiped from Gerard over at The Presurfer.