Invincibility

Start a mission, pause the game and press Square, Circle, L1, L1 + L2. This code must be re-entered on each level. 
Submitted by JUAN CARLOS LOPEZ

Level Select

At the Main Menu, press Square, Circle, R1, L1, R1+L1.
Submitted by JUAN CARLOS LOPEZ

Run faster in Multiplayer:

Hold Square + Triangle to chase your opponent more quickly. 
Submitted by JUAN CARLOS LOPEZ

All Weapons,

After you have started the mission, press the start button to bring up the pause screen. These codes must be put in within 2 seconds then press square,circle,R1,R2,then R1+R2.

Infinite ammo

square circle L1 L1+L2
Submitted by kokocat 




Tips/Hints

Slowly but surely

Walk slowly whenever possible. You can often spot enemy soldiers outside his cone of vision and take him out sniper style with one quick shot

Grenades Aiming skils

To get a higher chance of a kill, toss grenades slightly short since they have nasty habit of bouncing/rolling past your target.