You won't have a ripped back unless you have low bodyfat. Does that mean you need to train your back in a special way? The abs aren't magic muscles.
Keep the reps low, use weight, train them 1-2 times a week. They'll be strong and they'll look great when you lower the bodyfat.
In regard to situps, they aren't as great as one would think for the abs. Crunches are better. The one of the major points of the abdominal muscles contracting is to shorten the distance between the hips and the bottom of one's ribcage. Half of the situp does this, the other half just "sits up." Which half of the situp does this? You guessed it - the "crunch" half.
Do crunches instead.
-Warik