There is a big misconception when it comes to abdominal training. You often hear that in order to trim your waist line you need to do countless high rep sets of crunches and you need to train your abs frequently throughout the week. This is simply not true. You can’t spot reduce fat around your waist no matter how many reps you do so forget about it.
To have great abs you need two things. One is good abdominal muscular development and the second is a low level of body fat so the musculature is visible. Neither of these ingredients is accomplished with frequent high rep ab training.
For increasing abdominal muscular development you need to think of ab training like you do any other muscle group. If you want to increase development, you need to train the area with resistance. I recommend overloading the abs in an 8-10 rep range. A higher rep range is used compared to the normal Max-OT 4-6 reps so you can effectively overload the abs while maintaining good form and feel during every rep and maintaining tension on the abs. I'd say my favorite exercies for overloading the abs is cable crunches.
As for frequency, I think direct ab training once per week is plenty. Remember, abs get a lot of work all week long stabilizing your body during free weight compound movements so one time a week with direct overload should be all you need.
After increasing abdominal muscular development by executing intelligent Max-OT training you need to focus on the other ingredient for great abs which is shedding body fat and that is accomplished through attention to diet and intense Max-OT style cardio.
If you are smart and consistent with your diet and cardio, you’ll melt away the fat and the abdominal muscles you’ve worked hard to develop will be visible. The end result will be an awesome set of rock hard abs.