The three simple rules Valves' art staff used for creating the box art of the game Left4Dead.
The Rules of Thumb are:
1) There should be a hand in it somehow
2) The hand should have a thumb bitten off, so customers would look at the four fingers remaining on the hand and think, "Hey, four."
3) Under no circumstances would they use Photoshop. L4D is a visceral experience, and its box art deserved the same approach.