To get easy extra lives, deliberately get a low percentage kill ratio on a level 
where there is an extra life bonus. Anything below 70% kill ratio will force 
you back into the battle field to do the level again - with all the pickups and 
bonuses in exactly the same places!

Full Weapons 
While playing, pause the game and press Right, Down, Circle, Triangle, Up, Right.

Invulnerability 
While playing, pause the game and press Up, Left, Circle, Triangle, Up, Left. 

Skip Level 
While playing, pause the game and press Down, Right, Circle, Triangle, Down, Right.