What are some good exercises to strengthen the back muscles (i have chronic back pain from sitting down at work)?

"superman" exercise. Back injuries should always be checked by your doctor, especially if chronic or if one experiences symptoms down the leg(s). The superman exercise is ideal for strengthening the back muscles. Lie on your stomach, arms outstretched in front; lift the rt. Arm/shoulder and left leg/hip off the floor for 5 seconds, then repeat on the opposite side, do 12 sets, twice a day.
Depends on cause. Back pain can be from many different things, and there is no singlemagic bullet exercise to help them all. A therapy program (including home exercises) needs to be tailored to address the specific dysfunction. Other interventions may all help your back pain tolerate the therapy/exercises better. Get formal evaluation from pain doc, physiatrist, musculoskeletal doc to find cause of back pain!