To compliment the abdominals here are some exercises to help strengthen your back.
Single Arm Row
Standing beside the chair, placing your hand on the armrest for support. Step forward with your inner leg and back with your outer leg, bending forward at the waist and letting your outside arm hang straight down. Pull the dumbbell towards your waist, keeping your elbow close to your side. Keep your back straight and your head level. Start with no weights till you have the movement right, use a mirror or better yet a trainer who can help. U D
Do 15 reps on one side and then switch to other side. Once you have the move down, add weights ( or soup cans ), start with 1-3 pounds and work your way up. Also you can add more sets. For example, do 15 reps on with your left arm then switch to your right when you are done , do another set of 15 reps on each arm.
Low Back Strengthener
( if you have lower back problems check with your doctor or personal trainer first )
This exercise is very similar to The Superman. Begin by lying on your stomach with arms and legs straight out from the body. Raise either your right arm and left leg, or your left arm and your right leg simultaneously. Hold this position for about 3-4 seconds. Return to the start position. This is one rep. You can try 2 sets of 10 for this lower back exercise.
If this is your first time doing this exercise start with 1 set.