All Weapons
     Pause gameplay and press Square, Circle, R1, L1, R1 + R2.

Invincible Sarge
     Pause gameplay and press Square, Circle, L1, L1 + L2.

Level Select
     At  the Main Menu, press Square, Circle, R1, L1, hold L1 + R1 and
press R2.

Open All War Zones
     In Options, Difficulty, hold Triangle + Square + Circle and press
L1 + L2.

More Tanks
     At the Main Menu, quickly press Square, X, X, R1, R2.