Master Cheat Mode:
Go to the main menu and HOLD L and press 
Right, Right, Left, Left, Down, Up, B.  
This unlocks EVERYTHING!

Get Slow Deaths:
Go to the main menu and press 
Up, Up, Down, Down, X, Y, Y, X, Up, Up, Down, Down, X, Y, Y, X

Toggle Infinite Supers:
Go to the main menu and press 
Up, Up, Down, Down, A, X, A, X

Toggle AI:
Go to the main menu and press 
Up, Up, Down, Down, A, A, B, B, X, X, Y,