Go to the main menu, select the "Extras" option, then choose the "Cheat Codes" option. Find four boxes with pictures in them and line up the pictures in the correct order, then choose "Enter Code":
Invincibility: Baseball, Baseball, Baseball, Shirt
Big hair: Baseball, Bat, Bat, Baseball
Big head: Hat, Helmet, Helmet, Hat
Big feet: Hat, Glove, Glove, Hat
Sunglasses: Glove, Glove, Helmet, Helmet
Paper pants: Bat, Bat, Hat, Hat
Underwear: Hat, Hat, Shirt, Shirt