KT, just start off with regular floor crunches for your Abs. Crunches place less strain than sit ups and they are much more effective for abdominal work. Do you have the most recent edition of Muscle Media Magazine - it's a Training Guide ? They have some great tips for doing all workouts - including abs.

It's important when you do crunches that you squeeze your hamstrings and glutes, taking the hip flexors out of the equation - this isolates the abs and makes them work harder. The motion should be tight and controlled - you are trying to squeeze yourself together, not lift yourself up. You raise yourself to shoulder blades only. Breathing is also very important, as is keeping tension on the abs at all times.

Abs respond best to low reps with heavy weight - they are mostly fast twitch muscle fibers. Go as slow as you can, but don't be afraid to mix it up - slow one set, faster another, slow again, etc. If you feel up to it, you can hold a plate to your chest to increase resistance, but to be honest, I haven't found I need that - yet. I'm still getting a good workout w/o the added weight.

HTH
Nat