There are many people who spend so many hours trying to build up their abs so that could have that all-mighty six pack. However, many people fail to perform the exercise correctly and end up targeting their hip flexors instead of their abs.
First of all, I almost always recommend performing a sit-up or crunch on a decline slope. This is a slope in which your head is lower than your feet, and most gyms have these types of benches.
When you start out the exercise, try to relax all the other parts of your body except for your midsection. You can either cross your arms in front of you, put your fingers to your ears, or extend your arms straight behind your back. You should not put your hands on the back of your head as this may cause some damage to your neck if you start pulling on your head.
When you start the sit-up or crunch, make sure you do so in a nice, slow tempo. A lot of people go up and down wildly, and while this may cause you to become out of breath, it hardly works your abs. You want to make sure that you are not using momentum to swing your body up and down. The most important part is the eccentric movement. This is the part where you are coming back down from the top position. You want to make sure that you come back down very slowly and barely touch the bottom of the bench before coming back up.
As I said before, you need to make sure you are targeting your abs and not your hip flexors. You do this by curling up instead of coming straight up. The logic behind this is that you want to bend your abs because you want to work those muscles, you do not want to bend the waist only. However, once you have curled and cannot curl up any farther, you can bend your waist to get to the top of the rep. Doing sit-ups like this should drastically increase the difficulty, and you should not be able to do very many. This is good because most people do way to many reps of sit-ups. Hey, would you do 100 bicep curls per set? Didn’t think so.
Optionally, you can perform everything I said above on a Bosu or Yoga ball for less intensity:
The last tip I will give is that if you still can do a lot of sit-ups using the form and tempo described above, then I would add some weight from either a plate or dumbbell by holding it to your chest.