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. :)

