Beat the game in three hours or less without finding any of the people. You will get a rocket launcher and infinite ammo. Beat it with either of the people on difficult and get couger magnum and infinite ammo.
Beat the game with Chris or Jill and wait untill the credits, and the time it took you to finish the game are done and then a screen comes up showing you the special key.
Save the game and start a new one and find your way to the wardrobe room (which is shown on your map 1st floor) Open the closet door with your special key and walk to the end and press X to put on new clothes.
Jill gets a new hairdo and a cutoff shirt with blue jeans. Chris gets a leather bomber jacket and tan pants.
Cool huh! Now go kick some zombie butt in your new threds.