Arching my lower back as a default position for sitting and standing and avoiding rounding my back at all times. I always thought that slumped posture was the most important thing to avoid, but this created maladaptive muscle fixedness around the spine, confirmed by imaging taken by orthopedists.

The spine is adapted to bend multiple ways, including rounding, as a matter of good exercise form. It's a major function of your abdominal musculature, in fact, to compress the spine forward along the transverse plane. Just don't pick up heavy stuff that way or hunch over a desk all day.