The official rules of calling shotgun:
1) That person who calls shotgun first, gets to sit shotgun, no questions asked, as long as the following criteria are met:
1a) The person is not driving
1b) The person is outside of a building
2) If said person calls shotgun, but enters a building, they automatically forfit their right to sit shotgun. In this case, if while inside the building that person's friend calls shotgun (and he/she is outside), that friend is automatically awarded shotgun, even if the first person had called shotgun before entering the building.
3) If you are drunk and can't drive, you must still call shotgun to ride in your front seat (it's not your friend's fault you're shitfaced)
4) If any of these rules are not adhered to, you will get your throat cut by box-cutters by the person who is supposed to be awarded shotgun.