Unlock House Bots - On the competition mode menu press R, L, UP, UP, UP, L, R, DOWN, DOWN, DOWN, X.
Money Cheat - On the competition mode menu press L, R, L, R, Y, X, UP, LEFT, UP, R, L, R.
Unlock arenas - On the main menu press R, L, DOWN, DOWN, DOWN, L, R, UP, UP, UP, Y. 
Unlimited time - Pause the game then press R, L, UP, UP, UP, L, R, X, X, X, Y.
Invisibility - Pause the game then press R, L, RIGHT, LEFT, RIGHT, LEFT, RIGHT, LEFT, Y, X, X, X, X, R, L.
No armor - Pause the game then press R, L, R, Y, Y, Y, LEFT, RIGHT, LEFT, DOWN, Y.
Toggle AI - Pause the game then press R, L, X, Y, X, Y, UP, DOWN, UP, DOWN, R, L, R.