To unlock all campaigns and planes, hold down L2 and R2 at the main menu and quickly press square, L1, R1, triangle, triangle, R1, L1, square. (unverified)
To activate (or deactivate) God Mode in the game, pause your game, hold L2, and press the following: Square, Triangle, Triangle, Square. Then, release L2, hold R2, and press the following: Triangle, Square, Square, Triangle.
To increase the damage you deal to enemies (or to return to normal thereafter), pause your game, hold down L2, and press the following: L1, L1, R1. Then, release L2, hold down R2, and press the following: R1, R1, L1.