Originally Posted by
Chris Nagel
I agree with everyone here, but there's a few things I'd like to add. Start off your routine with exercises that emphasizes your lower abs, then do exercises that emphasis your obliques (e.g. side bends) or twisting exercises, then do exercises that stress your upper abs (e.g. crunches) last. The reason for this order is that if you were fatigue your abs doing exercises such as crunches first, you wouldn't be able to work your lower abs and obliques as much. Your abs still work all or nothing, there are no segments but the emphasis on particular areas vary depending on the exercise.
As for reps, first choose exercises that challenge you without being too difficult, this might be leg raises (lying down) to start with and and a couple of weeks later you can replace it with hanging leg raises. The number of reps should be about how much you can do in good form, you can also take a short break and then do another set. If you fully exercise your lower-abs and obliques first, your upper-abs will already have gotten a good share of work and you won't have to do as much crunches or whatever afterward.
I also recommend training your lower back as well, and doing stretches for your abdominals and lower back.
By the way, I have a good ebook that has a decent program for the abs I can send you or anyone else that's interested. It has different routines for different levels that you can follow. It's a nice progressive program. When the exercises get too easy, you can move to the next routine that would be more challenging.