If you just like to have a small, shaped waist and stomach or would like to strengthen your lower back, training your abs 3x a week, 1-2 exercises, 3 sets per exercise is all you should do. Even so in strength training for every muscle and every goal, like getting smaller or getting bigger, rehabilitation and improving bone density, high resistance/heavy weights is the answer. I am not a believer of heavy weights and high resistance when it comes to your abdominal training. Because your abdominal reacts faster it also has a tendency to get bigger and build bulk fast if worked to heavily (weight added to body weight) or worked too often (working abs everyday and getting bigger instead of smaller because the muscle develops under the fat). Unless you want to build bulk, train your abs without added weight or weight machines and work with high repetitions, 20-50 and up per set. Increase repititions and not resistance if it gets too easy. I lost 4 inches in my waist following the guidelines above.