The tried and tested Crunch: For this exercise lie down on your back to start. Put your palms on each ear with your wrist facing upward. Now pull your knees in to a comfortable position. Raise your head straight up, not toward your knees but toward the ceiling. When you begin to turn toward your knees you've gone to far. Tighten your muscles as much as possible then release. Try 3 set of 10 -20.
How do you know when it's working: When you feel a gentle pull in your abdominals and the muscles start to fatigue. Only your abdominals should be working. The neck should not feel tense and your back should be relaxed.
The Abdominal Plank: This exercise although simple is very effective at strengthening your entire core. All you need to do is start like you're going to do a push-up. Lower yourself to a position where you are resting on your forearms. Hold your back perfectly straight. This is a simple gravity exercise that works the entire core. Do this for as long as you can stand then do it 10 times.
You will know this exercise is working when you when you assume correct posture and find it difficult. Stand up straight and hold your gut in. If this is a little uncomfortable your doing great.
Reverse Crunch : This exercise works the transverse abdominis or lower abdominals and strengthens them. Lie down with your feet in the air or the knees can be slightly bent and gently bring your legs toward your abdominals. Pull the abdominals in tight. When the exercise is performed correctly, there will be a gentle pull in the lower abdominals. Keep the back on the floor fully and do not raise the neck or tense it. This exercise develops the V shaped muscles in the lower abdominals. Try to work towards 3 sets of 15 reps.
You will know the exercise is working when your lower abdominals start to hurt. If you have quite the gut then abdominal exercises can be difficult at first. You may find it necessary to jog or do sdome form of cardiovascular exercise to assist in dropping the weight. Just remember under that layer of fat is a six pack trying to show off.