NOTE: Everytime you go to a new extraction point you will need to re-enter your cheats

All Levels:
At the mission select screen hold down LT and RT, and the back button, and enter this code to unlock all levels: Y, RB, Y, RB, X

Full Health:
Start a mission, then press the start button. At this pause menu, hold down the left and right triggers, and the back button. (note, the back button is not "B", and it is not on the dpad. The back button in this cheat is the small one to the left of the xbox 360 guide button). While holding down these buttons, press LB, LB, RB, X, RB, Y. Then release the two triggers and the back button. If done correctly, a tone will sound indicating that the cheat is active.

Unlimited Ammo:
Start a mission and pause the game. Hold LT+RT+Back and slowly press: RB, RB, LB, X, LB, Y.
(The back button is NOT on the dpad, it is the button next to the guide)