My favorite, which I use all the time, is that it's bad luck to spill salt, and so to reverse the bad luck you have to throw some of the spilled crystals backwards over your shoulder.

I also don't open umbrellas indoors or walk under ladders. If I'm walking down the street with someone and a we're separated by a pole or something, I say "bread and butter". If the clasp of your necklace works its way around to the front, you can make a wish before you fix it (NOTE: having done this approx. 7,463,431 I can tell you that this doesn't work but it never hurts to try).