You can strengthen them by doing any one of these exercises:
1. torso twists (stay in a half sit-up position, arms extended with a medicine ball in your hand, then twist from side to side.)
2. side crunches (lay on your side with your hands behind your head and move your top elbow towards your feet, squeezing your sides. Remember to do both sides.)
3. Any other variations to sit-ups where rotation is involved.