No matter how good your posture is, if you're not moving there's parts of your body that aren't circulating blood as effectively as others. So one of the best things you can do for your back and the rest of your body is to make sure that you move. In the short and long term. 

If you can avoid sitting in the same position for longer than 20 minutes or any longer than it's comfortable then that can help, sometimes you need to actually change the way you are sitting, or even consider getting a yoga ball for an ergonomic chair.

Naturally it's also very helpful to walk around or move in some other way, it can be very helpful to have at least one time in your day where you get some physical activity, even if it's extremely mild. Our ancestors didn't evolve training for the olympics, many of them just set traps and picked berries, your body just needs you to get some physical activity on a regular basis.