Many times, the abdominal flaccidity has its origin in a bad body posture. Is this? The excessive curvature of the lumbar spine produces a prominent abdomen with consequent muscle flaccidity, abdominal here.
Excessive curvature is the product of poor posture, but also a shortening of the Para vertebral muscles, with the consequent stiffness and loss of flexibility. For this reason, it is not worth of nothing to do abdominal, if stiffness is still there, since sagging will remain in place.
Therefore, you can try stretching the entire muscle back and back of the legs. When you can achieve that, little by little all neuromuscular function will be reset and the abdominal will once again have its proper muscle tone.
In case, mentioned above, is not your problem, I suggest you perform aerobic exercises for lower body fat and then strengthen abdominal with specific exercises.