I don't believe in any superstitions. But there are some things I do just out of "tradition." For example, I'll "knock on wood" when I say something good that's happening. And I think it's more tradition for the bride and groom to not see each other before the ceremony as opposed to "YOUR MARRIAGE WILL SUCK."

EDIT: Black cats are supposed to be UNlucky.
Also, it's just common sense not to open an umbrella indoors.