It's not all diet. By building the ab muscles, obliques, etc... they protrude through the skin and are more visible. I'm not suggesting doing ultra heavy stuff, but definitely training them effectively and fatiguing them will cause growth. You have may noticed that some people have a low bodyfat percentage but no definition in their abs. It's sometimes becasue their abs are not developed.
I think plank and other isometric exercises are easier on your spinal discs (so they say) because you avoid flexion, but will not develop the muscles as effectively as dynamic exercises, such as bicycles, stability ball crunches, and hanging knee raises. (Change the muscle to quads-- would "wall sits" develop your quads as well as squats?).
Also, engage your core when you walk, move, and do other exercises. That will train the deep muscles to hold things in and become your new norm. If you don't know how, then pretend you're walking on the beach with no shirt on trying to impress a hot girl you're with--all the time--welcome to engaging your core!