To do effective sit ups you will first need to put out of your mind everything that you have heard about sit ups, and get with the new program instead. Although in days gone by it was thought that the most effective sit ups were those which you did by lifting your upper body and bringing it in towards your knees, today the outlook is a different one. Instead of lifting and bring in your upper body towards your bent knees, you instead lift only partway off the floor without really trying to come in towards your knees.
To do these very effective sit ups, you will need to get into the normal sit up posture, that is, you will need to lie down on the floor and bend your legs, keeping your feet flat on the floor. Bring in your bent legs as close as possible to your body without straining; place your hands behind your head with your fingers laced together, or keep them lightly on your torso.
To do the sit ups, you will need to use your abdominal muscles to lift your body off the floor about six to eight inches. If this is hard to do, try imagining that there's an invisible string connecting your belly button with the ceiling, and that every time you lift up, the string is what is lifting you up. You need to keep the string taut and straight, so you cannot bend forward, and you since your belly button is where this invisible string is, you can't use your neck muscles either to do these sit ups.
After a few reps of these you can begin to appreciate that these are indeed very effective sit ups. If you want more of a challenge however, or you just want to vary your routine somewhat to make the most of your need to do sit ups to get a tight stomach, there are variations on this type of sit up which you can do, and which are just as effective.
And one of the variations of these abdomen-tightening effective sit ups, is to simply include an exercise ball into your routine. Sit on the ball with your feet planted firmly on the ground, your legs about shoulder width apart to help you keep your balance. Maneuver yourself and the ball so that it is now underneath the small of your back, or in other words, you need to be lying on the ball with your torso parallel with the floor, and your legs keeping you balanced.
Place your hands behind the head and using the same muscles as earlier, crunch your ab muscles to pull your entire upper body between six to eight inches above the horizontal. To really feel the burn with these highly effective sit ups, when lowering your upper body back down again into position, go below the horizontal line by a few inches, before raising yourself up once more into the crunch. And Voila! you're done!