Knock on wood..
Touch Wood, An old superstition to avert bad luck or misfortune or to make sure of something good; also when feeling pleased with one's achievement or when bragging. Traditionally certain trees, such as the oak, ash, hazel, hawthorn and willow, had a sacred significance and thus protective powers. Properly these should be the ones touched, but this detail has largely passed into oblivion and any wood to hand is now used. Often, jocularly, the head is touched.
Brewer's Dictionary of Phrase and Fable