How about, when you say something at the same time as a friend, you're supposed to "link pinkies and make a wish".

The other form is to say "Jinx, you owe me a Coke" and as you get older "Jinx, you owe me a beer".

How about my mother's admonition against stirring things with a knife: "Stir with a knife, and you stir up strife."