It seems that the majority of negligent discharges happen during re-holstering. I have heard a couple of folks mention a "mode clear" or "mental reset" they do before holstering.
I'd love to learn more about the rituals folks do to make holstering safer so I can really incorporate it.
Previously I just straightened my trigger finger, and put my thumb on the hammer, then sweeping the top of the holster with my support hand, lifting my cover garment out of the way and holstered slowly.
But I realize that a pause before that, with a mental check, can help keep me safer.