Stomach exercises can be carried out in a number of ways, but if you do them poorly not only will they not do any good, they could cause a lot of harm. When about to start an exercise program of any kind you should seek advice from a professional, and each time you should include a warm-up routine.
Specifically with stomach exercises you should remember these following points; Keep Knees Up
Even though deep down you know you need to put in the effort, it's all too easy to cheat.
Whenever you are doing any type of crunches you should have keep your feet flat to the floor, knees pointed upwards as central as possible. This does make your crunches harder to do, but the results will be better as well. More importantly, if you do move your knees to either side it can compress your spine and this can lead to painful back injuries.
Sit-ups
No matter how widely they are used, traditional sit-ups do little to improve abdominal muscles, but the scope for them doing harm is immense. Even when done properly they do not exercise the abs, rather the hip muscles. Further, when people do sit ups they tend to do them quickly and the momentum rather than the technique is what takes over so no muscle groups are worked properly. The crunch, with its shorter movement is a better alternative.