I would say reverse crunches and cardio helped me the most! though it's not 100% perfect yet seeming I now have another boy 9 months old. but get a partner and lay on your back. put your head inbetween their legs and hold on to their ankels.Make sure their feet are close to each ear. pull up your legs (feet touching and legs slightly bent) and have them throw your legs back to the ground but dont let your feet hit the floor and pull your legs back up and repeat. I would do 12 reps and take turns. do 3 sets of 12 reps.
Have you tried Tracey Anderson's videos? I don't like the dance cardio, but her mat mathod and post-pregnancy workout yield amazing results.