You must say the word "Shotgun" to stake your claim on Shotgun. This must be done clearly and loud enough so that at least one other to-be occupant of the vehicle can hear you. No variations of this word are acceptable. After you have rightfully called Shotgun, you have exclusive rights to Shotgun for that ride. However, if no one hears you call Shotgun it is still fair game for everyone.
The website goes on to list the "official" rules as to when and where one can call shotgun and under which circumstances the rules do or don't apply. This little handbook (which can be purchased online in print form) would have been very helpful growing up. As a middle child in a family of three boys my mother often devised some fairly intricate systems for determining shotgun. The most memorable being a magnet letter of each of our initials that she would rotate on the metallic dash. I think that her methods were often fair, however easily disputable. Calling shotgun came into fashion in my teenage years (at least to my knowledge). This preferrable method, ensuring that, at any time, anyone could get the covetted position, was not without problems. When it is equatable to call shotgun, how to settle disputes, when is shotgun relinquished: all of these issues are discussed on the website linked above.
The primary place this would have been helpful would have been in high school. Luckily I was always the driver, as I had my own car and my two best friends, Eric and Brad, did not. The three of us would often travel places together and the position of shotgun, while officially determined by calling shotgun, was constantly in dispute and would result in Brad and Eric participating in a multitude of red-faced pushing matches. I can't deny that I usually enjoyed watching the two equally matched guys come close to blowing and o-ring over who sat in the front we were often late to things because of it. This website could have been very useful. Someday when I have kids I highly intend to get the glovebox version of the rules and abide by them. I encourage everyone else to do the same, thus spreading the universality of these rules and making car riding safe again. Well, as safe as you can be with a teenager behind the wheel of a car.