Invincibility
When your mission is loading (the little disc is floating in the upper right corner) hold down L1, L2, R1, and R2 until your mission has fully loaded. You should see a little box in the upper left corner that says "cheat." After you make 2nd Lieutenant, the pilots under your command will also be invincible.
Unlimited ammunition
Hold L1 + L2 + R2 + X during the entire loading screen with the helicopter that appears after receiving the mission orders. The word "Cheat" will appear in the upper left corner of the mission screen to confirm correct code entry.