Also take care to exercise important muscle groups for your job. Core strength helps a ton in sitting, and in my experience forearm adds a lot of stability to wrist and typing motions.
I would sometimes get tennis elbow (not sure why..) from desk work, and a specific type of releasing exercise was almost a light switch for the pain. So not just building muscle, but touching the right muscles in the right way i imagine is important as well.
If you're curious, specifically i use a type of rubber-ish solid tube that is designed for you to twist it. You can twist to build muscle in one direction, but you can also reverse your grip and "untwist" it under pressure. This tension-ed untwisting motion is what massively helped my elbow RSI, like a light switch, it was sort of insane.
That experience really solidified the idea to me that exercise is the foundation to damn near everything. Ie Physical Rehabilitation over reactive fixes like bone alignment / etc. Yea, posture is important, but i don't think posture can fix a body that isn't capable of supporting itself.