How can I get real washboard abs?

I’ve trimmed away all the fat now with my existing exercise routine. That’s a dumbbell lifting routine working everything except the mid-section, plus 50 each of crunches, leg lift and spread, and bicycle crunches to target the various abdominals.

I do this routine on even numbered days, and Heavyhands aerobic workout on odd number days. For the abdominal workout, I have 2.5 pound ankle weights I strap on to increase the work load a bit.

The routine above has got rid of all the midsection fat. All I can pinch now around my waistline is a bit of flesh. And the abs do show. The definition is there, but each individual ab muscle isn’t all that big. I’d like to pump that area up a bit. Without exercise machines to work the ab muscles harder, what should I do? Heavier ankle weights? Is their some alternative abdominal workout that stresses those muscles to exhaustion faster so I could reduce reps and build more muscle mass?