The best book for this is called 4-hourbody by tim ferris
I have a 6 pack and I do 6 sets weighted/seated crunches followed by torso rotations using a cable and then on the machine. Then, using the back extension device I lean sideways holding a weight and crunch my obliques alternating sides of course.
My belief is to confuse the hell out of the muscle so I switch up the order and the style. Maybe 3 weeks I use weight for the muscle to tear and develop then the next week I would do myotic crunch with a drop set of cat vomit exercise to lean it out. It has worked great for me.
Everyone has different body types though. I am a 6'6 225 ectomorph.
I used to be 180, but have worked to gain 45 pounds.
http://www.muscleandstrength.com/articl ... morph.html
for bodytype info
for abs info
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