A sack is when the defense tackles the quarterback behind the line of scrimmage (the yard line that you start the play on). Sacks are very easy to get in this game. When you are on defense, pick a defensive lineman as your player (keep pressing until you are one of the players that is closest to the ball). Move the player to the left or right of the offensive line so that no one is in front of you and you can run straight to the quarterback.
This can be done with two controllers. With controller two as the quarterback, just stand there so the defense is able to sack you easily.