The bird dog is a great exercise for improving balance and reducing back pain.
Start on the floor on all fours. Slowly lift your left arm and right leg simultaneously, holding both straight for 5 to 7 seconds. Do not raise your right arm or leg above horizontal; instead, stay parallel to the floor. Then switch sides.
Modified Version: If the full bird dog is too difficult, or if you have significant discomfort, try raising one arm or one leg at a time, and gradually build up to where you can hold both out, even if they are only partially rather than fully raised.
Advanced Version: Using ankle weights and a small hand weight, add 3 to 5 pounds per limb.
More Advanced Version: Start in the "up" position of a pushup. Holding that up position, lift your right arm and left leg simultaneously and hold for 5 to 7 seconds, then switch sides.