Beat the game in Easy difficulty to unlock the "Old Movie" cheat. This will make the gameplay turn black and white.
Crouch down for about five seconds and your aiming, especially with rifles, will become very steady. Once you stand up again and move, though, your aiming will start shaking.
If you are killed three times in a row, the game will offer you a chance to heal any injuries your squad might have sustained during combat. To expedite this process, you can just throw grenades at your feet.
Beat the game in Normal difficulty to unlock the "Infinite Ammo" cheat. Note that this cheat must be reactivated before starting a new level, or if the player is killed or retries the game.
Complete the entire game to unlock the "Super Squad" cheat.
In order to unlock the Authentic difficulty, clear the game on "Hard" difficulty. This difficulty has no aiming reticules, suppresion icons, and you cannot heal or revive your squad members by dying three times.
Create a new profile and name it BAKERSDOZEN and all the levels will be unlocked.