Bob super cute question! I have looked all over for the answers to this. I'm assuming you mean to prevent the bad luck, or whatever is supposed to befall you after a black cat crosses your path, but have found nothing. I did find, that it really depends on what country you are in, in some countries it is considered good luck for a black cat to cross your path. Here's some fun info: What's wrong with letting a black cat cross your path? Duh. A black cat might be a witch who transformed into the shape of a cat, of course!
"Cats fared badly (in the Middle Ages in Europe) ... as people thought they were witches' familiars. Black cats were believed to be witches in disguise," writes Richard Webster in "The Encyclopedia of Superstitions."
"An alternative belief was that after seven years of service to a witch, a black cat would turn into a witch. Consequently, a black cat crossing your path was an indication of bad luck, as the devil was watching you."
I grew up thinking its bad luck, but its really a mental thing. If you believe its bad luck then your going to convince yourself that bad things wil come. If you don't believe it then you won't have it in your head.
If a black cat crossed my path, while I was standing under a ladder, breaking a mirror, on the 13 th floor of a building, it would have nothing to do with what will happen to me. Because I always carry my lucky rabbits foot with me.