Here are five back exercises you can do at work to reduce the stress of long hours of sitting:
 
1.  Bird-Dog - Place both hands and knees squarely on the ground.  While keeping your abs tight and your pelvis neutral (no excessive arching or sagging), slowly lift the opposite arm and leg off the ground.  Extend both of them out while holding your head straight.  While lifting the arm and leg, squeeze your glut and do not allow your hips to tilt.  They should be square with the ground.  Do slow reps (10) at 2 -3 sets, two or three times per week.  You can simplify this by lifting only the leg.
 
2.  Swimmers - Lay flat on the floor on your stomach.  Fully extend your arms and legs.  While keeping your forehead flat on the ground, slowly lift the opposite arm and leg off the ground.  Do not bend the knee, imagine someone trying to pull your shoe off your foot as you lift the leg.  Your hips should be pressed on the ground and your gluts should be tighten.  Do not kick the leg up.  (The movement is similar to the doggie bridges)  Do 10 reps slowly at 2 - 3 sets, two or three times per week. 
 
3.  Figure 4 Stretch - Lay on your back.  Cross one leg over the other as if you were sitting at a table.  Reach through your legs and bring the crossed-leg position towards your chest.  You will feel this on the outside of your gluts.  Hold this stretch for 20-30 seconds, do 3-5 times on each side.
 

4.  Lifted hamstring stretch - Lift your leg onto your chair seat, desk, or table.  Square your hips with your shoulders.  Make sure your hips are parallel to the ground.  Then gently reach forward towards your shin while flexing your foot towards your shin. Hold this stretch for 20-30 seconds, do 3-5 times on each side.
 
5. Standing Rotation - From a standing position, place one leg in front of the other with a 1-2 foot separation.  Square your hips and shoulders with each other and keep your torso facing forward.  Gently begin to rotate your torso towards the leg that is in front.  Exhale as your rotate.  Come back to your starting position and exhale into the twist again.  Each time you twist, try to turn further, looking over your shoulder. Repeat on both sides 5 to 10 times.