Hit start to pause at any time during game play. Hold down L1 and L2 for about 15 seconds, until the red selection box cannot be moved anymore. Don't release L1 & L2 yet. While continuing to hold them enter the following codes:

Ammo:

Down, R, Circle, L, R, Circle

Health:

R, R, L, Down, Down, Up, Triangle, Circle

Power:

Down, R, Down, R, Triangle

Lives:

L, Down, R, Triangle, Square, X,Circle

Smart Bombs:

R1, R2, X, Triangle, Square, Circle, R1, R2, Circle, Circle, Square

Skip Level:

X, R1, Triangle, R1, Square, Circle, R2, R2, X, Square, Triangle, X

These items will now appear permanently on your pause menu. Pause and activate them with X at any time.

Walking Corpse:
Use the code to enable the health option on the pause menu. Then, go and get yourself killed. Just after you bite the bullet, pause the game and renew your health bar. Now, you can walk your dead body around the room you're in - and the meanies are powerless to do anything to you (of course, you cannot do anything to them either) This is especially cool on level 6.