NCAA Football 08

In the settings, turn off "Offsides" (which is actually done by adjusting the associated slider all the way to the left). Then, before the opposing quarterback snaps the ball, stand right next to him. Once he snaps the ball, tackle him. Sometimes you need to do this more than four times because if the computer was using a run play, it will count as a "Tackle for Loss" instead of a sack.

Make sure you don't touch the quarterback before the snap or you'll be flagged for encroachment.