The Office stretch everyone should be doing.

Sitting down for long periods
​[ anything more than 30 minutes at a time ] causes a chain reaction in our body.
Our hip flexors become slack , and so the brain begins to shorten them up to maintain the tension it craves.
The problem then occurs when we stand up again and our now shortened hip flexors pull our pelvis forwards, which in turn compresses the lower back and tightens the hamstrings. This often leads to lower back pain.

Here is a very effective stretch for releasing the tension from the hip flexors, when you have been seated for long periods:

Starting in a box position ,on hands and knees, step one foot forwards so the knee is above the ankle. Place your hands onto your front thigh and lift the body upright .
Push the hip forwards until you feel a stretch from the top of the hip down into the belly of the front thigh muscle, squeeze your buttock to tilt the hip further forwards to increase the stretch. Hold for at least 30 seconds and then change sides.