I think it depends somewhat on what 'kind' of abs you want: flat and lean like Rhonda on Cathe's videos (my preference) or the somewhat bulgy 'defined six pack' look of some Oxygen magazine cover models (can you tell that's not a look that appeals to me? LOL!)?
For both, you need to reduce any body fat that may be covering the muscle (definition is muscle tone or growth + low body fat).
For the leaner look, I'd go with working the abs more often, and adding core work like pilates or planks or kickboxing.
For the defined-six--pack look, work abs less often, and add weights.
For reducing body fat, diet is the key, and in general burning more calories through exercise (add in some interval cardio 1-2 non-consecutive days a week if you aren't already doing so, and do some circuit-type weight workouts that have somewhat of a cardio effect, like Jari Love's workouts) and the thermic effect of food (eating more often) and taking in fewer calories.