Add Bicycles, Leg-lifts, and change up your sit ups. Change your style of sit ups a lot. You want to be working every muscle, so break routine and try something different each time. I'm no body builder, but i've gotta stay strong for those nights where I have too many drinks. Haha. But seriously, don't just work for abs. Do pushups, too. Wides, diamonds, regulars, one handed, whatever. I've fought dudes who take their abdominal routines WAY too seriously, and mistake that as a sign of strength and merit. But they were dead wrong. Push-ups and hand stands are more important than what six pack abs will ever give you, I promise.
Also, i'd like to add that weight lifting is more show rather than actual substance. I promise you. Do calisthenics, don't lift weights.
