Google "posterior chain exercises". Practice them when you're not sitting down. There are many such calisthenic drills and stretches you can perform to prevent degradation of the muscles and nerve health in your glutes, hammies, lower back and calves.
Sitting down for long hours not only leads to muscle loss that can lead to nerve pains like sciatica down the line, it also downregulates nerve impulses in the affected areas which means that when you perform normal activities like walking/running/stairs, etc. you won't be activating those muscles to an appropriate degree relative to their antagonists (for example, when you squat, you would not be engaging the hamstrings and glutes in balance with the anterior muscles of the leg. the quads/hip flexors would be dominant.)
As a consequence of balanced muscles, your posture should correct itself over time and you'll also pay less attention to it because it will become something you're generally aware of and passively adjust as necessary.
Disclaimer: I'm not a physio. This is just experience + research so do your own as well.
