Inspect the door itself when you are checking out the other components. If the door is steel, look for rust spots that should be sanded, primed, and painted. Fiberglass doors can be washed with an all-purpose cleaner. Pay particular attention to wood doors, since warping and water damage are common. Remove chipped and peeling paint, then sand and repaint. If you have a wooden door that does not have weatherstripping along the bottom, make sure this bottom edge is well sealed or painted, then install a weatherstrip.