bah im not sure about doing it with weights, but ive had good success with bicycle crunches. You got mix it up thought try to 2-3 different excersizes per session.
You can check my gallery for results. All I did was very controlled crunches, so abs were fully tight during each rep. I did 5 reps, 10 seconds break, 10 reps, 10 seconds break up till just 25 reps. Then had a 1 min rest and repeat, and then one more min rest and repeat again. I then added dumbells in each hand, or a weight held on the chest. But really the results came without using additional weight and it was very easy to do.
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