When doing lots of crunches and sit ups on a regular basis, the normal expectation is to get some perfect abs with a flat stomach and a nice six pack. Even so, individual physiognomy and the look depend on a whole range of factors among which the genetic inheritance is not negligible at all. Normally, the abdominal muscles are covered by a layer of fat, which can be thinner or thicker from person to person. When the fat deposits are too large, the muscles will be impossible to notice, which is why the fight to get the six pack is in fact a weight loss challenge for many people.
Cardiovascular or cardio exercises combined with a good nutrition plan should support any training effort to get perfect abs. Aerobics works best for fat loss, and the activity is extremely beneficiary for the entire body system and the overall health condition. Although it is preferable to train all the main muscles, you need to target the abdominals with very specific exercises so as to get them better defined. Many people believe that the number of crunches matters most for perfect abs, but this is only partly true. Do as many exercises as you can, but slowly, while keeping the muscles tight. Both the lift and the lowering down should be performed slowly for maximum of efficiency.
It is equally important to be realistic in your effort to get the perfect abs. Do not trust all the commercials or ads with women and men that look almost artificially chiseled. Good looks are sometimes achieved by compromise and shortcuts that are not exactly healthy. Steroids and all sorts of supplements are often administered to stimulate the growth of the muscle mass. Normally, you should rely exclusively on the body's genetic response to exercising and not force it into growing artificially. Inflated 'perfect' abs are not the solution when you aim at good health and great looks.
As for the regular responsiveness to training, abdominal muscles are pretty easy to build by regular workout. Even so, the dimension of the results greatly depends on the individual predisposition, the nutrition plan, the rest between training sessions and the lifestyle in general. Harmonious muscle development goes hand in hand with healthy eating and sleeping habits and with a certain degree of relaxation and positive thinking. When working for the perfect abs, try to aim at something with a very positive connotation such as the improvement of the health condition, a better or longer life, preventing premature aging and so on.