Press L(2), R(2), X(2), Y(2), B(2) at the main menu. There is no confirmation message. Start a game then pause and enter the options screen. The "Cheats" option will be unlocked at the options menu.

Information in this section was contributed by Kturt133, Tidus147, and gun runner via Peter Adi.
Switch to a different soldier and remain idle (do not move, switch a weapon, or fire). The CPU will still be in control of that person. Note: The CPU will not move unless you switch during one of the commands. The CPU only fires at enemies.

Successfully complete the game under the hard difficulty setting to unlock the extreme difficulty setting.

If one of your teammates falls to the ground, throw a smoke grenade near him. Then, get your health packet out and heal him.

When you are moving ahead to scout a location, always leave your squad in a good position that they can cover you from, and in a pinch lay down some serious suppressive fire. Even if they do not hit the enemy, it will make them hit the dirt.

When you reach the third area where you see the first heavy tank, do not make any sounds. The tank crew is standing behind the tank. Have Conners and Foley kill them before they know that you are there, and before they can get in the tank.