> Finish the game on Easy difficulty.  Now, at the "Press Start Button" title screen, plug a controller into port D (four player slot) and press X, A, Y, B, B, B, Up, Up, Up, Up, Up.  You should hear a confirmation sound if you input the code correctly.  Now, when you start a game, all characters will 
have slightly bloated heads.
(courtesy: GameFan)

> Finish the game on Normal difficulty.  Now, at the "Press Start Button" title screen, plug a controller into port D (four player slot) and press L, Left, L, Right, L, Down, L, Up, R, Left, R, Right, R, Down, R, Up.  You should hear a confirmation sound if you input the code correctly.  Now, everything will be reflected during a game including the enemies.
(courtesy: GameFan)