You're right that just "doing abs" wont help much after some point. You eventually have to add resistance because they will have to have some reason to grow. Personally I got to where I was holding two plates on a declined bench doing sit-ups and was tired of that. If you have one of those sit-up machines then great. If not another option is to do shit lying on your back where you shuffle your feet.

But for me, I always developed my abs by doing other exercises. Like squat, deadlift (especially front squat). Do you think I could add 100lb to my deadlift without my abs becoming brutally stronger than they were before? So that's where I would bet my money on as far as ab training goes -- other exercises that require the whole body to move as a whole and the abs to bridge the gap.