There is no such thing as spot reduction

Abdominal exercises will not and cannot selectively burn the fat that may be covering your abs.

The abdominals do not attach to your legs and, therefore, cannot move the legs.

Leg movement that you may think comes from the abdominals actually comes from the hip flexors (iliacus, psoas, TFL and rectus femoris).

The abdominals, in general, are not any different from the other skeletal muscles; therefore, there is no reason to train them differently (e.g., no resistance, dozens of reps, several exercises in a given workout, training every day, etc.) if your goal is a change in appearance as opposed to increased endurance.

More specifically, the rectus abdominis (the main abdominal muscle that most people want to work for aesthetic reasons) is a phasic muscle, designed for fast, powerful movement.

To strengthen this and other phasic muscles, use weight that allows you to fatigue the muscle in about 20 - 60 seconds.

The abdominals are the major stabilizers of your midsection. Performing abdominal exercises early in the workout may jeopardize your ability to stabilize your body in other exercises, and may sacrifice the productivity of the other exercises in your program.

In most cases, abdominal (and low back) exercises should be performed near the end of the workout.

NEWS FLASH
The tip immediately above has been contradicted by recent research reported by the National Academy of Sports Medicine. My clients and I are currently experimenting with this new paradigm. I'll keep you posted on our results.