Most musicians when they sit on a bench/stool, slouch, which puts a lot of strain on the back, however if you look at pro musicians like piano/organ players they always sit upright to prevent these problems, therefore you need to get into the habit of always sitting upright if you want to solve the problem.
A seat with a back that has good lumbar support can certainly help, as it tends to force you to sit upright. (If you can get into the habit of sitting upright without help though all the better)

Bill