Back Extensions can be done lying face down on a hard surface, with a exercise mat under you. Place your forehead on the backs of your hands, palms facing floor, keep hands on forehead and raise the upper body a few inches off the floor. Legs and feet should remain on the floor.
Try doing the "Cat and Camel" too, This is where you are on your hands and knees, hands positioned directly under the shoulders, back in neutral (neither rounded nor arched) head should be in line with the neck, now hunch your back up, pulling the abs in, hold position for 10 seconds, then release, back to starting position. Repeat a further 4-9 times.
Try working on your core strength too, a strong back is reliant on strong abs and core. As someone else mentioned, Pilates is very good for back problems and wonderful for improving core stability.