This one might seem obvious, but it's one I wish somebody had said to me:
When you enter grad school, you're also entering a bureaucracy.
That means that there are meetings you must attend and a godawful amount of paperwork that you must do and all sorts of office business that you went into grad school to avoid, but alas didn't. Do everything you can to avoid departmental politics- they're poisonous. But, you cannot avoid the mindless form-filling. Not even by hiding in the library.