In the Cheat menu type Left, Square, Up, Triangle, Right, Circle, Down, X.
Invisibility:
In the cheat menu type Left, Square, Right, Circle, Left, Square, Right, Circle.
Slow-Motion:
In the cheat menu type Right, Square, Up, Triangle, Right, Square, Up, Triangle
Unlimited Ammunition:
While in the game type Down, X, Left, Circle, Down, X, Left, Circle.
Infinite Ammo:
While in the game press pause and type Down, X, Left, O, Down, X, Left, O.
Xen Gravity:
While in the game press pause and type Up, Triangle, Down, X, Up, Triangle, Down, X.
Alien Mode:
While in the game press pause and type Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle.