There is no such thing as upper and lower abs. The abdominals are
just one long sheet that run from about your rib cage to your pelvis.
You cannot work one part without working the whole thing. Many
times we cannot see an entire 6 pack, it is because we have excess
fat or loose skin over them. Belly fat tends to be lower, so we can see
the two "bumps" of the 6-pack. If you are doing reverse crunches,
make sure you are not using your hip flexors or momentum. The
reason we typically "feel" these exercises working in the lower
section of the abdominals is because these exercises use primarily
the iliopsoas (a hip flexor) which originates deep below the lower
portion of the rectus abdominis. This muscle is the one that fatigues
and because we feel it in the abdominal region, we assume we are
working our "lower abs" when we in fact are not. There is absolutely
no way to trigger a contraction in one specific area of the abdominal
muscles.

Taken from the ACE (American Counsil on Exercise) Fitness Matters newsletter written by
Chief Exercise Physiologist Cedric X. Bryant, Ph.D.
It takes a combination of strengthening
exercises and aerobics for great abs!

Crunches can only tone the abs. They cannot get
rid of the fat layer that lies on top of the ab muscles.
And the only way to lose the fat is to do some form
of  aerobic exercise like skipping, jogging, cycling,
taking aerobics classes, etc,  for no less than 30
minutes 3-4 times a week. These exercises are
imperative because they help increase the
metabolic rate, causing your body to use up all
stored forms of energy, in short, your fat.
Aerobic exercises burn calories/fat while
exercising, weight lifting build muscles, which
require energy just to exist, hence, you burn more
calories while at rest. So muscles increase your
metabolism, and enhance overall performance.
Ab exercise tips:

The walls of the abdominal cavity are supported entirely by the strength of surrounding muscles
since there are no bones that provide support for this region. To make up for the lack of skeletal
support, the three layers of muscles in the abdominal wall run in different directions. In the outermost
(superficial) layer is the
external oblique muscle, whose fibers run anteriorly downward and toward
the midline.  In the second layer, the fibers of the
internal oblique muscle run posteriorly and
downward.  These muscles are responsible for
torso rotation.

One way to perform this movement and work your
obliques is to lie supine (face up) on the floor,
knees bent, (you can also place one leg on the opposite knee), extend one arm out to the side on the
floor, and the other to the side of the head, elbow out to the side. Leading with the *shoulder*, lift the
torso up and over, in other words, aiming the shoulder towards the opposite knee.
Remember, it is not elbow to knee, it is shoulder to knee!  Eventually, you can add a weight to the
exercise to increase the workload and build more muscle.

The
rectus abdominis is a narrow, flat muscle on the anterior aspect of the abdomional wall that
flexes the vertebral column; it's fibers run vertically from the pubis to the rib cage. This muscle is
responsible for
forward spinal flexion.
Crunches are a good way to
start:

Again lying supine, knees bent, you can either cross your hands in front of your chest, or keeping
hands each side the head, elbows out to the side, keeping your abs contracted, lift the torso up
toward the ceiling, bringing your shoulder blades up off the floor. Exhale on the exhertion (as you go
up) & inhale as you go down. Eventually you can add a weight to that to increase the workload.
Many people keep their elbows in a forward position while doing crunches.  I personally prefer the
elbows out to the side for two reasons: 1) it makes it a little bit harder  and 2) it helps prevent you from
using your neck, which is a common mistake that most people make, the result is neck pain.

The
transverse abdominis lies in the deepest muscular layer in the abdominal wall.  The fibers of
this thin muscle run horizontally, encircling the abdominal cavity. It's only function is to compress the
abdominals, and serves
no motor function.

I do not recommend using ab rollers, or ab rockers to perform your ab work.  They are either useless,
or can present a risk of injury to the back, such as is the case with the ab rollers.  The ab rockers
make the exersice easy, and I only use them for clients with special conditions.

If you have an Exercise Ball, here is another way you can work your abs:

Lie face-up on ball and either cross hands over chest or place them each side your head. Make sure
that your knees are above the ankles and keep your torso parallel to the floor. In other words, don't let
your butt sag toward the floor, keep the gluts tight and your butt up so that you are, in effect, making a
"bridge" with your body.
Contract your abs and lift your torso slightly off the ball, pulling the bottom of your ribcage down
toward your hips.  For beginners, place the ball under the upper part of the back.  For advanced, the
ball should be under lower-mid back.

Vertical Leg Crunch:

Lie flat and press your lower back to the floor. Put hands behind head and extend legs straight up
with knees crossed.  Contract abs and lift your "tailbone" toward the ceiling. Keep abs flat throughout
the movement and remember not to hold your breath!

As described above, there's no such thing as upper and lower abs. The rectus abominis is not
divided into two parts. The entire muscle will be worked when performing spinal flexion.
Spinal Flexion is not the same as Hip Flexion. In other words, doing leg lifts while crunching does not
work the abdominals more effectively.  Leg lifts and bicycling work your
hip flexors. While your abs
may contract in order to stabilize the torso, mere torso stabilization is not the most effective workout
for the abdominals. Stick to crunches and increase the resistance with weights as you are able.           
                              
    
            
    
Rectus Abdominis
If using a weight
while performing
crunches, balance
the weight on your
chest with your
hands crossed
over your chest and
holding the weight.
If you have a
pre-exisiting back
condition, I do not
recommend using
weights while
doing crunches.
Always obtain
your doctor's
consent before
beginning any
new exercise
program.