Please note that self-help is generally illegal in many states! 
What makes it my seat?: I have claimed adverse possession of it from the law library. I have changed its state by rearranging the chairs, have been open and notorious about it being my seat, can exclude the library from it (with the use of signs and/or Supreme Court Reporters used as barricades), have held it continuously (while studying, with no long gaps while studying), and am hostile and adverse about anyone else taking it!
I have also been studying way, way too long, I think. 
I was wondering how long it would take for someone to take the joke dead-seriously!