1. The basic rule: If you are the first to see the car and holler "Shotgun" you may walk slowly to the car, smug in the knowledge that your seat in the front is reserved. It doesn't matter if you are not actually first to arrive at the car. What matters is who hollers first, not who gets to the car first!
2. You must be able to see the car before you can holler "Shotgun!"
3. If no one hollers "Shotgun" and you get your hand on the door handle, you are awarded shotgun automatically.
4. No pre-hollering. In other words, the errand has to be over and you have to be on your way back to the car before you can holler "Shotgun!" Example: you and a friend are getting out of the car on the way back to a store. You cannot holler "Shotgun for the way back!" This is laughable. A corollary: knowing that the car is parked behind the big black truck, or around the next corner, is NOT same as actually seeing it.
5. Window sightings. Illegal. You cannot "see" the car through a window and holler "Shotgun!" You must be outside in the view of the car.
6. If you leave shotgun briefly (a gas station stop, for example) you still have shotgun rights but the clocks starts ticking. You have 5 minutes. After that, the position is open. Other riders wishing to claim shotgun must holler, as usual, from outside of the car.