Karma ... keep working on your best asset, then.
For my tum, I mostly do crunches. If you try them sitting on a Swiss Ball, it keeps the tension on your stomach, and doesn't stress your back.
Then I do leg raises, lying on my back.
And a core stability move that looks easy, but is a killer, called The Plank. Lie on your front, and prop yourself up on your elbows, as if you were watching telly. Then tighten your tummy, and pull your tummy up off the ground until your body is straight, and resting on your elbows.
It's a killer !! ... but it works.