If you create a profile called BAKERSDOZEN, it will unlock all of the levels in the game.

Clear Easy difficulty to unlock the "Old Movie" cheat. This basically allows the game to be played in black and white.

Clear the game on normal difficulty to unlock infinite ammo. This cheat needs to be "reactivated" at the start of each level, if the player is killed, or he retries.

Clear the game on the hardest difficulty. This mode has no checkpoints, saves, icons nor reticules.

Play and complete the game on Authentic Mode to unlock the super squad cheat.

Simply beat the game once on Easy. Then go to chapter select and play the very last level on a different difficulty level this will award you with that cheat.