At the main menu press L1, L2, L1, Circle, Square, Circle, Circle, L2, L1, R2
Access All Tanks
At the main menu press L1 + L2 + R1 + R2
Double Shields
At the main menu press L2, L1, R2, Circle, Square, Circle, Circle, L1, L2, L1
Invincibility
At the main menu press L1, L2, L1, L2, R2, R1, Square, Circle, Square, Square
Secret Level