Exercising. Losing the baby fat and toning your muscles will improve your abdomen but there is almost always some extra skin and fat left in the lower abdomen.
Loose skin. The best solution is a tummy tuck but it is a fair amount of surgery. It has the best capacity to tighten though it does leave a scar in its wake. :).
Varies. For a mild case exilis radiofrewuency has good results in a out 4-6 weekly visits. For more severe cases surgery is necessary- tummy tuck.