Interesting topic. I do knock on wood and believe in jinxes. In 2004, I wore the same clothes while watching all the Red Sox playoff games against the Yankees once they started winning, and then continued wearing them during all 4 World Series games until they won! Because of my outfit!
And then, Ben Affleck was on Regis and he told the story about wearing his same lucky clothes and not changing until they won the Series! How many of us wore the same clothes watching those games, and worried about jinxing it if we changed? And how gamey did we smell by the final game???
I found some interesting possible origins of Break a Leg:http://en.wikipedia.org/wiki/Break_a_leg
I think for me the jinx stuff all stems from a kind of OCD - I found out I have a form, I'm a Counter. I count how many times I rub on deodorant under each arm, (God forbid I don't apply the same counts under each!) or how many times I brush blush on my face on each side, or eye shadow, etc. I had no idea I did it until a friend spoke of her son being a Counter. Whoa, something else to worry about now.