The single most important area of the body from both a functional and aesthetic stand point are the midsection or abdominal.  Yet a good number of people even experienced exercisers have a tendency to leave their abdominal exercises until last in their program or workout.
This is a mistake for three primary reasons:
1. For most of us improving our midsection, reducing our waist line, or building those washboard abdominals is or should be a priority so why would we leave it to last. 
2. Training your midsection first in the program serves to warm up those stiff vulnerable lower back muscles and focuses unconscious attention on maintaining good posture thought-out the rest of the workout.
3. When we leave our midsection training for last, we either don′t do them at all or we half-ass it and ultimately making for a weak and unattractive midsection. 
There is an unlimited number of midsection or abdominal exercises that can be performed and yet another common mistake many exercisers make is using the same basic exercise over and over again. If I see one more guy doing sit ups I′m libel to slam him with a right hook. The midsection must be worked from a variety of angles and variety of exercises in to maintain structural balance.
Although we often hear the “abs” referred to as two separate components the upper and lower abdominals they are not.¬† The rectus-abdominus which starts at the base of the diaphragm and attaches at the pubic region is one large muscle.¬†
There are however many different components within the “midsection” or core that support or facilitates movement from a variety of angles. Think about how much directional movement we have with our trunk.¬† We can bend forward and backward, we can rotate, twist, bend, and stretch from right to left and back again.¬† The only real limiting factor in our trunk movement is our spine.¬†
The weaker the midsection and lower back muscles are the more restricting or stiff the spine becomes, which is one of the most important reasons of all to maintain the integrity, strength and balance of the midsection. 
The key to developing and maintaining the structural balance and integrity of the anterior of our entire midsection is in working it through all four of its basic directions while challenging the deeper more difficult to reach core stabilizer and postural muscle.
Add these three movements to the beginning of your training program and watch your trunk become as solid as on oak:
Anterior Crunches on the Stability Ball:
Begin by laying back on the ball your knees shoulders width apart and at a right or 90 degree angle.  Hands under your chin. Once in position begin by curling your trunk upward toward your knees while pushing your chin toward the ceiling.  keep your hips and buttocks elevated or perpendicular to the floor as if you were lying on a bench.  Make sure you exhale on the upward contraction and inhale on the way back.  Use caution on the ball, go slowly and you will feel an even deeper burn.  Perform 2-3 sets of 20 -30 reps.
Combo Crunch:
The purpose of the combo crunch is to recruit both the upper and lower regions of the abs simultaneously.   Begin in the floor crunch or sit up position and then bring your feet off at a right angle to your knees.  Once in position begin by contracting your trunk and curling your elbows and knees inward simultaneously so that they meet and touch in the middle.  Exhale as you curl in and inhale as you go back and momentarily relax the muscle.  This one will give you quite a burn.  Perform 2-3 sets of 20-30 reps
Russian Twist:
While seated on a bench, or on a fit-ball begin by grasping a 8-10 lb medicine ball or dumbbell between your hands.  While keeping your elbows bent at a 90 degree angle and tucked tightly against your sides, hold the ball out in front of you at about bellybutton level.  Begin to rotate either right of left keeping your head and eyes forward at all times.  Exhale as you rotate away from center and inhale as you approach center.  Be sure not to over rotatedo the movement slowly and unforced while tightening your midsection as you rotate buy blowing out your air as if you were taking a punch to the ribs.  I generally recommend a slight pause in the middle as you rotate form either right or left in order to keep yourself from gaining too much momentum during the full rotation.  2-3 sets of 50-100 reps