I'm no expert, but I have very defined abs.
I train them with weighted rope crunches or machine crunches once a week and on another day i do leg raises and bodyweight decline situps in sets of 40-50 reps. I do most of this for Muay Thai training, but it helps keep the abs rock hard as well.
I honestly think my abs were built by years and years of heavy deadlifts and squats, but the exercises i do today simply keep them in shape since i don't lift heavy anymore. My abs haven't changed much in the last 5 years, but i do remember them becoming more pronounced after a few years of heavy power lifting.
Just my .02.