my dance teacher used to make us do this! Exercise for the Transversus Abdominis (AKA the muscle that pulls in the ring of your waist) One area of your abs you may be neglecting is the transversus abdominus - without working it, you run the risk of developing a square torso (straight up and down) rather than an hourglass one (tapered at the waist).
After baby (even if baby is now 4 years old). This workout will tap into the deep abdominal muscles that will pull in your waistline like a corset. Do these moves one after another with no rest in between. Then repeat once.