Posture is really important for desk-work and can often result in injury, even from wrong posture or micro-movements, or lack of movement!!!
It might be more than just your back so check out your whole body posture. Sometimes you get referred pain or tightness from something else. Here is a link to an ergonomic article about setting up your desk space. Remember, it could also be the angle of your neck and the height of your screen, the angle and support for your feet, the angle of your elbow to the desk or the angle you hold your mouse on. It can seem a bit pendantic until you set a new habit. I also try to get up and move around regularly, or stretch regularly although I tend to get absorbed and time flies by. Check out the link = you don't necessarily need a fancy chair, but these tips might help iron out some kinks you have!!
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http://www.spine-health.com/wellness...duce-back-pain