Invincibility:
Press L2, L1, R2, Circle, Square, Circle(2), L1, L2, L1 at the main menu.

All tanks:
Press L1, L2, L1, Circle, Square, Circle(2), L2, L1, R2 at the main menu. All
tanks, including a super tank with unlimited weapons, will be selectable.

Bonus level:
Press L1, L2, L1, L2, R2, R1, Square, Circle, Square(2) at the main menu.

Double Shields:
Press L1, L2, R1, R2 at the main menu