I have a study where I have a desk with a computer which I use to write my hubs. Recently I learned that it is important to create a writing space and a sort of routine to begin writing. Of course, correct seating is important because I too have had back and shoulder problems. A friend, occupational therapist, suggested that we have to stand up and walk around for 2 minutes for every 20 minutes of sitting down.
One way to do is to have break reminder software. Check out my article on Free Apps to Avoid Eyestrain for some suggestions. Another resource is juststand.org in which you can get information on the disadvantages of sitting for too long and the remedies.
We usually get carried away when in front of a computer. Another suggestion is to use a kitchen timer.