All weapons and missions:
Press L, Black, White, R(2), White, Black, L at the main menu.
Unlimited ammunition:
Press Start to pause game play, then press R, L, Black(2), White, R, L, Black, White. Repeat this code to disable its effect.
Invincibility:
Press Start to pause game play, then press L(3), Black, L, R(2), L, Black. Repeat this code to disable its effect.