Press the Back button during the game, then enter the following codes:

Code - Effect
A, X, B, Y, A - Big Heads
X, X, Y, A, B - Chicken Expolsives/Bombs
X, A, Y, B, X - High Pitched Voices
X, X, A, B, A - Individual god mode
Y, Y, B, X, A - Slow Mo Mode
B, A, Y, Y, A, B, X, X, X - Team God Mode
B, A, X, Y, A - Two Dimensional Mode - when you look at the other soldiers on your team, they will be two dimensional