It's something I kind of took for granted before, but now I'm really wondering, after having watched Mac Lethal try to perform "Lookin Bro" live and realizing he didn't remember the lyrics...how the fuck does Eminem do it?
The only time I've ever seen him forget/screw up on stage is during one of his performances on the Warped Tour.
It's not even so much him remembering the songs on his albums that impresses me. It's that every time he goes to the radio station to do a "freestyle" (pre-written verse), he absolutely nails it. He even mentioned after he did the "Swag Juice" stuff that he and Mr. Porter had just come up with the ending on their way there.
My best guess is he just listens to everything he records so obsessively that he knows all his verses like the back of his hand.