You should treat your abdominal muscles like any other muscle group, which means you shouldn't train them every day. Your abdominals, like all of your other muscle groups, need recovery time between workouts.
According to Cedrix Bryant, Ph.D., Chief Exercise Science Officer for the American Council On Exercise, you should ideally want the last few repetitions to be difficult to complete.
Performed correctly, 10 to 25 repetitions for one to three sets of abdominal exercises provide a more than adequate training stimulus. If you can perform more than 25 repetitions of an abdominal exercise, you are most likely performing the repetitions too rapidly or with improper form.
You can increase the challenge and intensity of abdominal exercises by using added resistance, moving more slowly or performing the exercises on a slant board or exercise ball so that your head is at a lower elevation than your legs.