Hello! I have that issue sometimes. I've found that the triple twenty rule works pretty well--take a break every twenty minutes to stretch, look away from the screen for twenty seconds (farther away the better), and blink twenty times. Also, Gunnar Optiks sells computer glasses that cut down on screen-related eye strain. I have a pair and they really help when I have to be painting for long periods of time.
For posture, I try to keep my back straight and legs uncrossed while working. Doing some aerobic exercise in the morning before working also helps prevent back strain for me. Hope that helps!