Ditch factory seats in favor of lighter "racing" seats. Add a 4 or 5 point harness and ditch the air bags, associated parts and sensors.
If you are really serious, strip 100% of the interior, ditch all motors in favor of mechanical devices (door locks, window motors). Next would be to replace all the windows with TFAL Flexan or other lightweight glass alternative. These modifications are not for daily driven vehicles.
