I only train my abs with resistance. Medicine ball throws, overhead squats, heavy rope pull downs/leaning pull downs, spread eagle situps with a plate or med-ball, etc...
Sk*, I found that I have to do WAY too much volume for my abs to "feel it" too. Then I got to thinking, if any other muscle in my body were getting bigger/stronger, I'd up the weight when it got easy. Why would my abs be any different?