Even if you ignore all other rules of elevator etiquette — you cut the line, talk loudly on your cell phone and travel a grand total of one floor — do not break this one: Face the doors.
Most people can deal with standing way too close to someone for a few seconds, if that someone's face is pointed in a neutral, parallel direction. If you turn around and put your back to the door, it creates an awkwardly confrontational standing situation. Or, your fellow riders might think you're from another planet.
The whole "facing forward in the elevator" thing apparently came from the mid-1800s when elevators had a back row of benches [source: NPR]. (This seems like a custom that should be revived). The only exception to this front-facing rule might be standing with your back against one side of the elevator, but only in a noncrowded car situation.