Press Select while playing, highlight Help then enter the code. Disable the code by entering it again.
Infinite health:
L2, L1, L2, Square, Circle, Square, R3, L3, R3, Circle
One-hit kills:
L2, L1, L2, Square, Circle, Square, L3, R3, Circle, Square
Complete level:
L2, L1, L2, Square, Cricle, Square, L3, R3, L2, L1
Power mode:
L2, L1, L2, Square, Circle, Square, R3, L3, Circle, Square
Infinite ammo:
L2, L1, L2, Square, Circle, Square, L2, L2, L1, L3
Tiny characters:
L2, L1, L2, Square, Circle, Square, L3, R3, Square, Circle
Big characters:
L2, L1, L2, Square, Circle, Square, R3, Square, Circle, L3
Unlimited Phase Cloak:
L2, L1, L2, Square, Circle, Square, L1, R3, L2, L3
Big head:
L2, L1, L2, Square, Circle, Square, Start, Square, Circle, Start
Character select:
L2, L1, L2, Square, Circle, Square, L2, L2, L2, L2
