Big heads:

Press Back then press A, X, B, Y, A during game play. A message will confirm correct code entry.

Chicken explosives:

Press Back then press X(2), Y, A, B during game play. A message will confirm correct code entry.

Flat characters:

Press Back then press B, A, X, Y, A during game play. A message will confirm correct code entry.

Slow motion:

Press Back then and press Y(2), B, X, A during game play. A message will confirm correct code entry.

High pitched voices:

Press Back then press X, A, Y, B, X during game play. A message will confirm correct code entry.

Hint: Medals:

Purple Heart: Get wounded or killed in action.
Bronze Star: Get 15 kills
Silver Star: Get 20 kills
Distinguished Service Cross: Get 25 kills
Congressional Medal Of Honor: Get 30 kills