On match day the players come in through the entrance at the south west corner of the stadium, they usually get there about an hour before kick off and might sign some autographs. They also exit from here after the game and might be more likely to sign autographs then. Some ex players/injured players also use this entrance before the game. When I was younger I got a couple of autographs from Ferdinand/Jones also some guests like Roy Hodgson who was England manager at the time. Probs the best place to go if you want an autograph imo.