Lie on the floor, feet on the floor, knees up and fingers placed behind your ears or hands crossed on your chest. Sit all the way up, lifting your lower back off the floor along with your shoulder blades.Lower yourself down. Repeat. Once this becomes relatively easy for you (i.e. you can do a quite a bit with ease) start adding more challenges. Find an incline bench or do these with an exercise ball. Once you "graduate" from that, do weighted sit ups. Hold a weight on your chest while you do these. As these become easier, hold heavier and heavier weights.