Everyone that has to ride with friends has at some point had occasion to call "Shotgun". For those of you who are not up on the full list of rules for calling shotgun, I've included a few official amendments to the original shotgun rules.
Amendment XXXIX: Shotgun Suicide If the Shotgun caller attempts to open the car door as it is being unlocked (thus causing it to stay locked), he immediately loses Shotgun priviliges for the upcoming ride, and a new round of calling Shotgun must be executed.Amendment LII: The Rock Amendment
This rule states that once a passenger calls "Shotgun," he must also say, "No Rock." If the gunner does not say this, another passenger may call, "Rock." In this case Shotgun is awarded to the winner of a best of three, Rock, Paper, Scissors contest.
You can find all the rules, along with their amendments, at http://homepages.nildram.co.uk/~starbug/shotgunguide.htm.