Remember, the abdominals stabilize the rib cage and aid in forced expiration. Exercise the upper abs before the lower abs. Therefore, it would be a mistake to pre-fatigue the hip flexors first because you'd then weaken an already weak link thereby limiting the effect of the exercise on the upper abdominals. Perform all exercises in good form. Avoid hyperextending the spine.
People frequently complain of low back pain while executing abdominal exercises.
This is usually the result of having relatively weak lumbar extensors, performing the exercise incorrectly or a combination of the two. This will help keep your lower back flat, thereby reducing the amount of stress placed on it during the performance of the exercise. Keep tension on the abdominals throughout the entire duration of the exercise. So, its not necessary to bring your torso all the way up to your legs. In addition, don't let your head touch the sit-up board between reps.