While playing press BACK (next to the X-GUIDE button) and input the following codes: (Unverified)

Load up ... UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, RED, GREEN
Restore life ... RED, GREEN, RED, GREEN, UP, UP, DOWN, BLUE
Restock ammo ... RED, GREEN, BLUE, YELLOW, LEFT, RIGHT, LEFT
Invulnerability ... DOWN, UP, DOWN, DOWN, UP, RIGHT, LEFT, RIGHT, YELLOW, BLUE, BLUE, GREEN