the recommendation these days is not to do full on sit ups where you go from lying down to sitting up as it puts too much strain on the neck, etc. The correct way these days is to go for more of a stomach crunch.
- Lie on the floor with your legs bent, your feet flat on the floor. If you prefer, you can keep your legs bent at a 90 degree angle so that the bottom of your feet are vertical. Engage your stomach muscles.
- Place your hands on the back of your head, or if you prefer, cross them over your chest.
- Lift your shoulder blades off the ground moving your elbows towards your knees. Don't try to lift your whole back - just your shoulder blades.
- Squeeze your abs for a second.
- Slowly return to the starting position.
- Repeat steps 3 - 5 until you've completed the desired number of reps.