From the American Council on Exercise comes this advice on how to correctly do the best and most equipment-free exercise for abs: the bent-knee sit-up crunch:
Lie on your back on a mat
with your knees bent, feet flat on the floor and heels 12 – 18″ from
your tailbone.
Place your hands behind your head, squeezing your shoulder
blades together and pulling your elbows back without arching your low
back.
Exhale, contract your abdominal and core
muscles and flex your chin slightly towards your chest while slowly
curling your torso towards your thighs. Your feet, tailbone and lower
back should remain in contact with the mat at all times. Continue
curling up until your upper back is lifted off the mat. Hold this
position briefly.
Gently
inhale and slowly lower your torso back towards the mat, keeping your feet, tailbone and lower back in contact
with the mat.
Read more from the American Council on Exercise.