It doesn't work to do ab exercises, at least not after a couple months. What does work is to pick up heavy stuff, do pullups, pushups and planks. 10 months of abs twice a week (it wasn't endless crunches, but there was usually a set of situps or crunches in with the other stuff) only made me a teensy bit stronger. Two weeks of really trying super hard to do a pullup plus doing planks, squats and military presses among other things and I swear I can see my abs way better and I can feel they are getting hard. Two weeks, only 6 sessions total. That's all so far and I see progress.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Heaviest squat: 180 x 2. Heaviest Deadlift: 230 x 2