> Pause the game, then hold L and press the following:

99 Heavy Ammunition
A, B, X, Y, B, A.

All Maps
A, Y, X, B, Y, X.

Fix Toy
A, X, B, Y, A, Y.

Heavy Weapon
X, A, Y, B, A, X.

Hidden Single-player Level 
A, B, A, B, Y, X. 

Switch Machine Gun
B, A, Y, X, A, B.

(courtesy: GameFan)