On the title screen, press B, B, B, A, A, A, X, X, X, X, X, X.
At the character selection screen, press Select to edit your controller configuration.
Kick some serious shell in two new levels with this trick. At the title screen, press L, R, L, R, L, R, A. Now select Vs. Mode and you'll be able to fight in the Metro Train stage and the Studio 6 stage.
At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A on Controller Two.
Play as a Boss
Use this trick to play as any boss. Plug in a second controller, wait until the title screen appears. On Controller Two, press X, Up, Y, Left, B, Down, A, Right, X, Up. Now go to Vs. Mode and press Left on Controller One. You can now play as any boss.
Use Super Moves in Story Mode
At the title screen, press Up, Left, Down, Right, X, Y, B, A, X, Y, B, A, X.