Pennies on the ground - if they're 'heads up', I pick them up for good luck and give them to the next person I see. Heads down, I leave 'em alone.
Whereas when I find heads-up pennies, I stick them in my left shoe (opposite of my dominate hand) overnight to change the luck. Tails-up pennies brings a wish.
If you want an explanation for this--heads up means the spirit granting the wish will know who you are and might jinx up your wish.
Spilled salt - I throw a pinch over my left shoulder. I don't even know why.
Salt is a cleanser and a preserver and, while a necessity of life, was often quite expensive. If you spill salt, it's bad fortune. Thus, you throw a pinch of it over your shoulder and into the eye of whatever bad influence is lurking there.
And I do this too, without thinking about it.
When I got married, and when we renewed our vows, I stuck to the 'something old, something new, something borrowed and something blue' advice. Again... I don't know why.
Something old signifies the couple's honoring of traditions and family values.
Something new signifies the couple's new life together.
Something borrowed signifies the community's part and commitment in the couple's life together.
Something blue signifies good fortune since blue was an expensive dye in days gone by.
Then there's also knocking on wood when you're saying something positive or negative about self or future or plans. (Again, that's to deafen the wood spirits from jinxing good fortune or playing into doubts.)