These posts are kind of funny to write, as my belly is still anything but flat two-years after a late-in-life C-section. I am still confident that I wouldn't enjoy the strength (and healthy back) that I do if it's weren't for dedication to Pilates and yoga.
My other favorite belly flattener is a variation on yoga's "Dead Bug" pose. I just added some lower body movement to challenge the internal obliques and lower rectus abdominis (that would be the "six-pack" in super-lean folks).
This really targets the "Abercrombie muscle" -- or the low, low abdominal muscles that you see on the jean's posters in the Abercrombie & Fitch stores. :)