I'm betting there are plenty of core exercises you aren't doing, but today we're going to talk about the BEST one you're not doing.
I've taught exercise for fifteen years now, and I have seen people crunch, flex, strain, bulge, and jut just trying to get their stomachs in shape. And most of that effort was in vain. The natural tone of the abdominal muscles will kick in when the pelvis and ribs are in the right position. It's true! Here is another fun fact. The strength of the abdomen always matches the strength of the back. In trying to maintain "good posture" we tend to pull the shoulders back and slightly arch the lower-mid back, creating tension and stiffness in the spine, just above the waist. Now you have a tight and weak back, with over-stretched and weak abdominals on the other side, or what I like to call "a back spasm in the making".
When we think of a strong stomach, we usually picture the long, up-and-down "wash-board" abdominals (rectus abdominus), but it's actually your oblique and transverse abdominal muscles (they run right to left and on diagonals) that give you a waist. If you don't have good waist definition, a crunch can actually widen your middle. Not what you had in mind when you bought that last 3 Minutes to Better ABs VHS, I know.
Looking trim in the mid-section is a great bonus, but the true benefit to a waist that is equal to or less than your hip measurement is the stress it removes from your cardiovascular system. The smaller arterioles in all your muscles should be holding their share of blood, reducing the pressure in the larger arteries. The tighter the muscles, the less blood they hold, and the higher your arterial blood pressure. The abdominal aorta is most susceptible to plaque accumulation because the pressure there tends to be very high. It tends to be high because most of us have a very tight mid-section as a result of the excessive time we spend in hip flexion (sitting), the lack of rotational movements we have in our daily lives, and the tension we have in the back (holding ourselves upright with the spine instead of the glutes and hamstrings). Tight, larger abdominal muscles (obliques and transverse) can't aid your cardiovascular system in they way they should - which is why a larger waist size is a risk factor for coronary heart disease.
To start whittling your waist and decreasing the pressure in your arteries, try a simple spinal twist.
Lie on your back and bring one knee towards your chest. Try to take (don't force it!) your knee to the ground, leaving the rest of the body where it started. If you waist is very tight, you won't twist as much as roll to get the knee to floor. You can tell you rolled if your spine and shoulders all rotated over with your hips. Ideally your shoulders and chest all stay planted on the ground and your waist muscles are long enough to allow your pelvis to move. If you are a "roller", do this exercise every day until you start to twist, decreasing waist size, increasing the strength of the stomach and back musculature, and decreasing your blood pressure. Make sure to do both sides. (This is also a great exercise to improve your golf game too!)