Distractify has come up with the 22 rules for calling "shotgun." The rules are even broken up into 5 sections! Some of the rules are:
1) When 2 or more people call shotgun at the same time, the first person to touch the passenger door handle wins
2) Once shotgun has been called, the back left and right seats can be called, leaving the slowest person stuck in the middle
3) Anyone calling shotgun must have shoes on to prevent people from running outside to call shotgun and having to go back inside to put shoes on & slow down the journey.
4) "Shotgun Suicide"--When the shotgunner attempts to open the door just as the driver is unlocking it & jams the lock half open so that the driver needs to unlock it again. The shotgunner forfeits their position when this happens.
5) The driver is in control of the music, but if they feel the road requires their full attention, duty is passed on to the shotgunner. Putting on crap music or allowing for silence when the iPod finishes a song will result in demotion to the dreaded back middle seat.
Check out the rest of the rules here