I have a desk job and I spend my entire days in front of my computer. About one year ago, I've traded my desk chair for a ball commonly used for yoga, pilates, gym... It has really decreased my lower back pain. And it's also a relief for my entire spine since I don't lean back on any backrest (which is something that causes me pain on my spine). I also think that it makes me sit more straight that on a desk chair.
It might be worth trying, those balls are usually not very expensive and you can find them in most sports shops.
I also use a very low stool (about 13 cm-high, 5 in) to put my feet, I feel better than when I put my feet directly on the floor.
This is the link to the balls I'm taking about: