If you have a genetically thin waistline and a strong abdominal wall, I recommend maintaining it. This can be done without multiple sets and reps. People forget that the abs come into play on every exercise. If you are squatting, bench pressing, shoulder pressing, rowing, curling, or any other exercise. . .you are training the abdominal wall to a degree. Here is a simple routine that Follow that might work for you:
1. Pulley Crunches - 1 x 40
2. Incline Crunches - 1 x 40
3. Reverse Crunches - 1 x 40
4. Twisting Crunches - 1 x 40
Performing this twice a week is enough. If you want to improve muscularity. . .watch the diet. The main thing is keep it simple.