By exploiting an easy to learn AI weakness, you can score a sack on nearly every play in Madden 2004.
When on defense, pick the Free Fire blitz from within the 4-3 package. Move your two interior lineman away from the ball one click each, move the lineback on the left down, and move the middle linebacker a little bit to the right and a little down. When the offense snaps the ball, the left tackle will go to block the linebacker on the left, the left guard will block the defensive end on the left, and the center will block the middle linebacker. This will give the interior lineman on the left side a huge hole and have a free path to the quarterback, which is a tactic that the offensive AI will hardly ever account for and can easily lead to several hundred sacks throughout the course of a season.
For even better results, sub in one of the fastest players from your defensive backfield into the position of the lineman that will have a free path to the quarterback so that you can sack him that much faster.
Thanks to being able to use any playbook in the game on offense as well as the ability to line up any player at any position, you can easily use this to your advantage and move the ball virtually at will.
Regardless of what team you're using, choose the Washington Redskins playbook and then put your best wide receiver at fifth on the wide receiver depth chart. When on offense, run the Slants play from the five wide receiver Shotgun formation. This will cause your best receiver to either be matched up against one of the worst defensive players from the other team or to be wide open outright on virtually every play. The defensive AI will never figure this out, and you can literally run this same play over and over again until you score whenever you have the ball. In the rare event that the defense drops back or adds extra defenders, simply Hot Route your best receiver and have him run a curl route; he'll always be wide open due to always being against one of the backups from the other team.
This obviously works best with a team like the Vikings or Colts (who have Moss and Harrison on their teams, respectively), but the glitch is so cheap that you can use it with virtually any team in the NFL and have easy success.