Pause the game and press R1, CIRCLE, R1, L2, R1, L1, SQUARE.
Quick Gun
At the start of the game, run though the back door to a gun faster and safer than going to the man at the front. Hit him in the head and he will die faster.
Better Sniping Accuracy
When in sniper mode, zoom in. To steady yourself for greater accuracy, sit there and do not touch anything to look or move for about five secounds or until he stops moving around. You can now look around with perfect dead aim accuracy.
More Ammunition
Wait until an enemy reloads his weapon before killing him. Kill him before he fires his gun and you will get a full magazine for your weapon.