Between 10 and 25 repetitions of sit-ups should prove effective for most people. The number of sit-ups one should do in a day depends on a person's physical strength and endurance level.
During the third or fourth repetition of the exercise, sit-ups should be noticeably more difficult. A feeling of fatigue is a sign that the exercise is indeed working the abdominal muscles. Thus, the amount of sit-ups must be adjusted periodically according to muscle tolerance. Proper technique when doing sit-ups is also important to maximize effectiveness. The body should be on a flat surface with the knees at 90 degrees. The feet should be flat on the ground, and the hands should be below the chin or near the ears before beginning sit-ups.