1- High-intensity of muscular overload
2- Progressive intensity from workout to workout
3- Proper spacing of workouts to avoid overtraining or under training
Most people do sit-ups or crunches as an ab exercise. While these are basically good exercises that can satisfy point 1 above, how many people use them in a way that satisfies point 2? Muscles will only develop in response to overload that is above normal. So if you do 20 crunches every day for a year, why would your ab muscles develop beyond that capacity? They won't.
To force new development, you need to increase the intensity. You could add a few crunches every day, but that really just increases duration; there is a better way to get fast results.
Abs exercises proven to work
1- Weighted crunches
2- Weighted incline sit-ups
3- Weighted sit-ups
The best way to do weighted crunches is to lie on the floor with your head close to the low pulley weight stack. Using the rope handle attachment, grasp the ends and pull the cable until it is tight and your hands are resting at the side of your head near your ears. Now contract your ab muscles in a crunch that lifts your shoulders off the floor and draws the weight stack up an inch or two. Choose a weight that is so heavy, you can only do 8 to 12 reps.
If you don't have access to a low pulley, there is a good alternative. You can use the high pulley that is normally used for lat pull downs. Kneel on the floor or sit in the seat directly under the rope handles that you attach to the high pulley. Lock your legs under the hold down. Pull the handles into position next to your ears, then contract your ab muscles into a crunch that raises the weight stack an inch or two. Again, choose a weight that is so heavy, you can only do 8 to 12 reps.
As a further alternative, you can lie on the floor and do a sit-up or crunch while holding a barbell plate against your chest. The limitation of this exercise is that, as you progress in strength, it will not be possible to hold enough plates on your chest safely. But that's a good problem to have.