You clean up your diet to lose weight, now do the same for your workout. Those mindless crunches you can crank out for minutes at a time are the "junk" in your ab routine. If you can get past 25 reps with any move, your probably not engaging your middle muscles. I'm not saying that crunches are bad, but you need to do a variety of moves-with focus and intensity-to flatten and firm your belly. The exercises in pilates will do the job by targeting your muscles from all angles with resistance.
Anatomy Lesson: Your abs include four muscle groups that work together. To flex your spine, you use the rectus abdominis. Twist or stabilize your torso and your external and internal obliques along the sides of your middle kick in. The transverse abdominis compresses your midsection and engages when your obliques do.
Flattening your middle is a three step process, you have to eat healthy, do cardio, and work your abs. Do only the latter and it will take you much longer to see results.
I can't wait to help you strengthen your core, and by doing so flatten and firm your belly!!
In good health!! Kelly