When you stand up and feel are abdominal muscles, you'll find something interesting. Although you're not really trying, your abs (and much of your core muscles, both superficial and deep)are contracted lightly. There's a reason for this. They are all lightly contracted in order to provide structural stability to your lumbar spine. Unlike your upper back, there are no other bones to provide support (think: ribs). Granted the lumbar vertebrae are far bigger and thicker than the others but they still need a lot of support of the core muscles. This includes the spinal erectors, the abs, obliques, etc.
This light, gradual contraction gives us a direction to point our abdominal training. The majority of our core muscles are slow twitch muscles due to their light but constant effort.
So, most of the exercises that we do for the abdominals should be of a moderately difficult but sustained period of time. This is why exercises like the pushup and pulllup are so good for conditioning the abdominals: You have to contract them in order to keep the body in a proper position while performing the exercise. Also, aerobic isometrics are good too. Many of the higher volume calisthenics will work well as well.
Now, I'm not saying that this is all that you should do. I certainly do some wheel rollouts, Farmer Burns Stomach Flatteners, and isometrics for the core. I'm just saying that exercises that require prolonged contractions are probably best for maintaining spine health. You get this from the above-mentioned exercises.