Invincibility:
At the Loading screen (when you see the graphic of the helicopter), press L1 + L2 + R2 + R2 simultaneously and hold Down until the loading screen is vanishes. As soon as the mission screen appears, you will see the word "Cheat" in the upper left corner.

Infinite Ammo:
At the Loading screen (when you see the graphic of the helicopter), press L1 + L2 + R1 + X simultaneously and hold down until the loading screen is gone. As soon as the mission screen appears, you will see the word "cheat" in the upper left corner.

Alternate Views:
During game play, press L1.

GameShark Codes

Infinite Ammo 800a429803e7
              800a429a03e7
              800a429c03e7
              800a429e03e7

Infinite Fuel 800a42aa0064
 

Thanks to Revolution readers Nutbunny, KinglyDuckBoy, Joey Sharp, Cia20637, and Brian Burita Jr!