I read Lisa Sutton’s ab training article on Bodybuilding.Com and thought it was very good and agree on the sequence of training the abs: first, lower abs; second, side (obliques) abs; and last, upper abs.
When training the abs keep in mind that the main function of the abs is to "flex," i.e., round, the lower back, not to arch it or twist the spinal column like in seated twists or side bends. Therefore, any proper ab training is/should be a "back flexion" movement even when working your "love handles" or oblique muscles.
Technically speaking, the ROM for working the ab muscles to a fully contracted position is a 2-inch movement. Try it yourself, but only flex the lower back. Any more than that means involvement of hip flexion and back extension, which should be minimized in order to maximize your time doing ab training. To MAXIMIZE back flexion pull your rib cage and pelvis in toward one another at all times (no hip flexion and back extension). Remember the 2-inch movement ab rule!
The main goal is to bring the ribs and hips closer to each other or to close the gap between the ribs and the pelvis. A person can only do that if the abs are consciously pulled in and tightened as in any crunch position. When performing sit-up crunches from start to finish try and keep your neck in line with the upper body by looking up (not forward and tucked down) or place your hands behind your head for neck support to avoid hyperextending the neck and possible neck pain.
Avoid Low Back Pain!
When the abs are weak the lower back tends to arch rather than flexing at the hips to work the abs. We may do this without realizing it, especially when the abs fatigue. At this point, most people start relying on the lower back for strength or support. The result might be low back pain, if not now then later. If you have low back pain conventional lying leg raises should be avoided and replaced with reverse crunches.
When performing reverse crunches for your lower abs keep your lower back snug against the pad or floor. This exercise is performed by raising your hips (not your thighs) up and back while pulling your abs in. Bring the hips up and in (not your thighs) to perform a “back flexion” movement. The same motion is also true with hanging leg raises, except that by lengthening of the spine might make it more difficult to perform a “back flexion” movement. To avoid this, pull yourself up to a 1/3 pull-up position by your lats for more stability and less momentum and pull your hips forward to get yourself into a curling or rounded back position.
Quality NOT Quantity
Perform two sets for the upper, lower and side (oblique) abs. Remember, you are aiming for quality not quantity. Accomplishing more in less time means quality, not how much or many. So it might be beneficial if you do not count your ab reps, but instead aim for complete muscle fatigue per exercise performed in a tri-set fashion. My motto, “train hard, not long,” also applies to ab training, not just weight training. Abs should be trained slow and concentrated since they contract fast throughout the day during daily activities. The contracted position should be held for a tight 2-second count. Try to avoid using momentum as much as you can and also avoid using the strength of the legs, hips and ankles to reassure yourself that you are getting a good quality ab workout!
Best Time and How Often?
Abs can be done as part of your initial warm-up at the beginning of your workout or be used as a warm/cool-down last in your workout. It is can also be used as an intermission between muscle groups worked if you are following a split routine. Ab training should be performed three to four days per week – not everyday.
One more important thing…
Most people train their abs doing countless sets (even with weight) in hopes of reducing the fat around their waist quicker. This is a mistake. What you must do is perform compound movements like squats, dead-lifts, standing military presses, standing curls, etc. because these exercises involve large muscle groups of the whole body by using tremendous amounts of energy! Compound movements build muscle and hasten the “after-burn” after exercise!