Destroy All Humans!

Pause game play, then hold L2 and press Square, Circle, Left(2), Circle, Square while on the Mothership. All upgrades for the currently unlocked weapons will be available for purchase from Pox's lab.

Every chance you get, do not kill the human before you extract their brain stem. Extract their brain stem first as it will make the DNA worth more (25 instead of 10).The easiest humans to do this to are the scientists in the level with glowing cows. For some reason, if the person that is important to you dies (the person you need to abduct), extract their brain stem before you return to the Mothership -- the DNA is usually worth more.

On the mother ship hold L2 and press R1, R1, R2, R2, Left, Right, Left, Right, R2, R1. This gives you extra DNA.

Shoot the tires of any vehicle with the zap-o-matic instead of the main vehicle body and their health goes down faster.

Hold L2, enter cheat, then release L2.

Invinciblity
Square, circle, left of d-pad, left on d-pad, circle, square

Infinite Ammo
left on d-pad, circle, R2, right on d-pad, R1, square

Infinite Mind
R1, R2, circle, right on d-pad, R2, circle

Aware Like A Fox
Right, Square, R2, R1, Right, R2

Nobody Loves You
Right, R2, R1, Square, Right

Mmmm Brains! (Must be entered on Mothership)
R1, R1, R2, R2, Left, Right, Left, Right, R2

More Upgrades (Must be entered on Mothership)
Square, Circle, Left, Left, Circle, Square