Everyone’s always asking “How do I get a flat stomach?” or “How can I get a six pack?”
I thought I would take the time to write some information that should answer most questions and/or give them some useful knowledge.

Lets start by dispelling some of the common myths about abdominal training:

• Ab workouts require 50-200 situps to be effective.
  Wrong. Your abdominal region is made up of muscles, just like your chest, back, or legs so why would you train them any differently.

• Doing situps or other various abdominal exercises alone will give me that six pack I crave.
  Wrong again. If it were that easy guys with man boobs would have six packs and girls with huge butts and thighs would have them also. It just doesn’t work that way.

There are 29 muscles that make up the core region. The two most popular will be overviewed here.

• Rectus Abdominus - This is the most common area, also referred to as the "six pack." However this is not six separate muscles, its one large muscle with raised fiber areas. RA also helps stabilize the hip complex. Rectus Abdominis

• Obliques - These muscles are located near your rib cage. There are four oblique muscles, inner and outer left and right. The inner help with body posture and both assist in trunk rotation (twisting). Fitness Equipment, Workout, Fitness Program, Fitness Articles, Beauty Fitness, Weight Loss, Weight Training

Some other important muscles to know are:

• Transverse Abdominus - This muscle is located under your rectus abdominus, and mainly pulls your stomach in, giving it that flat look. The TA also helps stabilize the lumbar spine and hip complex.

• Erector Spinae - This muscle runs the middle of the back, under the latisimus dorsi and trapezius. It provides assistance in stabilizing the spine during movement. Erector Spinae

So what is it gonna take to get the thin waist or bulging abs that everyone seems to desire?

• You need to reduce your bodyfat.
  ---To reduce body fat you need to be eating a well balanced diet and incorporate a full body weight training and cardio routine. Diet is of utmost importance if your goal is to have a well defined midsection.

• You need to start burning more calories than you are consuming.
  ---To do this safely you need to find your daily maintenance calories. Once this is achieved, you need to reduce it by about 20% or 500 calories. If you are burning this extra 500 cals per day, then over the course of 7 days you should have lost 1lb (3500 kcal).

• Do not overtrain your abs.
  ---Incorporate 1-2 direct ab exercises into your routine. If you are doing a split setup, than work abs once a week. The core is being worked in all your other exercises, you just don't feel it. The core stabilizes all movements. If you are on a full body routine, than do abs last with 1 exercise per workout day, no more. You can switch in exercises to keep yourself entertained as well.

• Diet, Diet, Diet.
  ---Your diet needs to be 90% spot on. Everyone is allowed cheat meals. I believe Dr. Berardi when he talks about the 90% rule. He states "The difference between 100% and 90% adherence to your diet is negligible." Noone is absolutely perfect, but you need to be pretty darn close. If you're still trying to figure out a good diet, or are constantly messing up, then you're gonna have major problems achieving that thin waist.

So what are some exercises you can do to train your abs. Here ya go:

Bridges
Cobras
Roman Chair Leg Lifts
Crunches
Long Lever Crunches
Ball Cobras
Ball Crunches
Ball Long Lever Crunches

Lower Abdominal Myth

It is widely believe the lower abs are exercised during the leg raise or other hip flexor exercises. It can be misleading to judge the mechanics of an exercise based upon localized muscular fatigue. The primary muscle used in hip flexion is actually the Iliopsoas, one of many hip flexors. The Iliopsoas, indeed, does happen to originate deep below the lower portion of the Rectus Abdominis. During the leg raise the entire abdominal musculature isometrically contracts (contracts with no significant movement) to:

Posture the spine and pelvis
Supports the weight of the lower body so the lumbar spine does not hyperextend excessively
Maintains optimal biomechanics of the Iliopsoas
Hips are kept from prematurely flexing if the lumbar spine and pelvis does not hyperextend excessively
Iliopsoas can contract more forcefully in a relatively slight stretched position
Bent knee (and hip) situps actually place Iliopsoas in a mechanical disadvantage
Counteracts Ilopsoas's pull on spine
Many people with weak abdominal muscles are not able to perform hip flexor exercises without acute lower back pain or discomfort
The combination of the local muscular fatigue, or a burning sensation from the isometrically contracted abdominal muscles, and from the working hip flexors produces fatigue in the pelvis area which we mistakenly interpret as the lower portion of the Rectus Abdominis being exercised. In movements where the Rectus Abdominis does Isotonically contract (contracts with movement), it flexes the spine by contracting the entire muscle from origin to insertion. The spine is not significantly flexed during the leg raise. Incidentally, both the spine and hip flexes during the Sit Up and Hip Raise.
From Fat Loss & Weight Training Myths