Pause the game, hold the Left Trigger, then enter the following codes and once entered release the Left Trigger:

Code - Effect
Right, X, X, Y, X - All Weapons
Up, Right, Up, Down, X - Alter Lighting
Down, Up, X, Y, X - Buddy Decals
Left, Up, Right, Down, X - Fish Eye View
Up, Left, Right, Up, X - Invisible Mode
Left, Down, Up, Right, X - Slow Motion
Right, Up, Up, Down, X - Wacky Fade