Thou shalt not let your opponent see the ball closest to your pocket.
It took me a long time to understand why people said this, and when I didn't understand it I way underestimated its importance.
It isn't because you want that ball to stay close to your hole. You do want that, but it's less important than the real reason, and thinking this way is bad for your tactical game.
It's because that's the ball your opponent can hurt you the most by playing off of. It's generally 2 or 3 times easier to put your opponent in a vicious trap by playing off the ball closest to his pocket than it is by playing off any other ball. You let a skilled and experienced opponent see that ball and you won't like your next shot. You follow this commandment and one pocket will involve a lot less suffering.