At the main menu, select the "Extras" then choose the "Cheat Codes" option. Line up the pictures in the correct order by scrolling them, and press "Enter Code" to activate them.
Hat, Glove, Glove, Hat --> Big feet
Baseball, Bat, Bat, Baseball --> Big hair
Hat, Helmet, Helmet, Hat --> Big head
Baseball, Baseball, Baseball, Shirt --> Invincibility
Bat, Bat, Hat, Hat --> Paper pants
Glove, Glove, Helmet, Helmet --> Sunglasses
Hat, Hat, Shirt, Shirt --> Underwear