I agree with lean tean, crunches are more effective and work the abs better. I find that sit-ups are almost useless for abs, because as you come up over the 30-45 degree angle, your back muscles and hip flexors take over anyway.
In his early days Bruce Lee did:
* Waist twist - 4 x 90
* Sit-up twist - 4 x 20
* Leg raises - 4 x 20
* Leaning twists - 4 x 50
* Frog kicks - 4 x 50
If Bruce was tired, he would do only three excercises and less reps.
I altered this to suit me:
* Waist twist (with flexing) - 4 x 90 --A broom is used as stabilizer for rotational movement!
* Sit-up twists - 1 x 20
* Oblique crunches (weighted) - 1 x 25 (ea. side)
* Leg raises - 4 x 20
* Side bend streches (with flexing) - 4 x 20
* Regular crunches (weighted) - 2 x 20
* Bicycle crunches - 1 x 50
* Standing curl crunches (w/flex) - 2 x 20
The last is the hardest. To do it: Stand up right, feet a little more than shoulder width
apart, hands on hips, come back as if looking at the ceiling, then going to a downwards position down as far as you can to the groin area while there flex real hard--you will feel it.
It works your midsection, and sometimes I can feel a nice burn all over my abs while doing this. Take it easy, start with small movements and avoid usage of the back. Try not to rest longer than 90 seconds in between workouts. Try not to work your abs everyday, but rather 3-4 times a week. If you have abs and you can't see them you will have to start doing more cardiovascular excercise and significantly reduce carbohydrate intake (especially those unhealthy processed carbs).
Bruce Lee also taught his students that spot-reducing was just a notion, and that ab excercises will build muscle, not reduce the fat surrounding them (that is the duty of cardio and dieting or eating healthy).
Good luck!