You might want to try picking up an exercise ball and using that to sit on while you work. I use one at work, and it makes it so much easier to sit up properly. Not only that, it makes you use your core muscles, rather than just your back muscles, and so lightens the load on your back and helps your abs at the same time. *grins* The size should be such that the angle of your knees should be just a little more than 90 degrees.

And when you aren't working, Elizabeth can play with the ball. :-)There's nothing like having a bouncy ball bigger than yourself to play with.