sitting isn't a very natural position for humans. squatting is supposed to be
i used to find sitting really comfortable and lacked the mobility to squat. now i've got the mobility to squat (maintaining a good lumbar curve) i find sitting really very uncomfortable for my lumbar spine.
one thing to try is a lumbar support cushion. i think the idea is for it to encourage you to sit into your glutes / hip flexors more like a squat rather than collapsing / rounding the spine into the seat. breaks are good. moving around generally.
i actually do most of my computer work lying down these days with a laptop. no one position is comfortable enough for me to stay in it for very long, so i fairly continually move without thinking of it. when i'm seated i kind of forget to move until things start hurting.
makes driving hard, though.