If you find that your back is feeling stiff the longer you sit, an easy way to make sure that doesn't happen is to keep your back moving, your body moving, while you're sitting.
Of course, back stiffness is rarely caused by just 1 thing but at least this exercise can eliminate 1 of those things!
Drop your elbow towards your hip then elbow towards the other hip, getting your body moving side to side. You can also do a twist so your shoulders twist on your hips and do that occasionally throughout the day. You can also let your pelvis tip backwards and forwards.
Just going through all the different motions will ensure your back doesn't get stuck in the same position by sitting still for too long.