Hidden options:
Highlight the "Training mode" selection on the options menu and press L1 & L2 & R1 & R2. Additional options, including invincibility and stage skip, will now appear
Play the Same Character in Two-Player Mode:
To get this trick, both players must choose a different character, leaving the character that you both want to play un-chosen. During gameplay, have player one press START to pause, then SELECT. Choose the Change Character option and select the desired character. When you return to the game, player two must immediately do the exact same thing. The characters picture should still be there and will be selectable--you have to be quick though.
Screenshots
