Shotgun

It's happened to all of us before: you're out with a group of people, and you start heading back to the car to go somewhere else. You glance at your fellow passengers and loudly call "Shotgun!" Everyone knows you've laid claim to the front seat. However, especially in cases with siblings, there can a lot of fights over the rules of calling shotgun. I felt it was time to lay down some guidelines; you can follow 'em if you want, but all I know is they seem to make sense to me.

Okay, first off, rules for when you can call shotgun. Shotgun may only be called when at least one of the following conditions is met:
a) The vehicle to be used is in sight
b) It has been decided to return to the vehicle, and the group is in the process of leaving to the vehicle
c) Notification has been received and given to the group that someone will soon be arriving with a vehicle (i.e. a taxi)
If shotgun is called before any of these conditions have been met, the first person to point out the error will be awarded shotgun. If nobody notices the error, the caller receives shotgun.

Rules for Challenging Shotgun
Once someone has called shotgun, another person can take their privelige from them by completing any of these steps:
a) Calling "Blitz". Once blitz is called, all those wishing to obtain the front seat must run to the vehicle. The first person to touch the vehicle is awarded shotgun. Note: Blitz must be called immediately after shotgun is called.
b) Pointing out an error in the calling of shotgun if one occurred.
c) If the caller attempts to open the passenger door while the driver is unlocking it, causing it to remain locked, the first person to point this out before the driver successfully unlocks the door is awarded shotgun.

That's all I can think of at the moment, but feel free to post some of your own shotgun rules in comments and I'll add them in!