Now, where did I park again?
For long-term parking, SFO recently switched from a big lot to a 7-story parking structure. At one end of each floor is a bank of elevators and on the ground floor is the bus stop to get to the terminals.
Each floor is (somewhat subtly) color-coded and right next to the button for the elevator is this little widget: a card with the floor printed on it, and a space for each of the sections that can be marked or torn to indicate where you left your car. Simple, elegant.