Build up Rage Meter with soda
Smash any pop machine untill cans fall out, press X while standing over one to drink from one. It will build up your Rage Meter.

Capitalize on the stupid AI
In any level where there are desks that you can climb/jump onto and your enemies are not armed with any type of gun that they can shoot you with this trick will work . It's simple, if you are iin a room that meet those conditions just climb onto a desk. Your enemies will stand still, allowing you to take them down with YOUR guns.

Invincibility, infinite ammo and stage select.
At the "press start" screen press and hold these buttons: L1 + L2 + R1 + Triangle + X + Left. If entered correctly, once you "start game" you will be brought to a level select screen. You can choose to continue your old games also, they will have the code effect as well. Once in the game, all your guns will have unlimited ammo when they are picked up and you will not die. But you may have your health deducted if you fall from high places or burn yourself. If you fall into a 'pit'. You will still die, so be very careful at edges. Use the walk button (L1) at the edges. It will prevent you from falling. (Warning- You will not be able to save at all)