Start at the shock manufacturer's recommendations. Only change one setting at a time, and don't change damping more than 2 clicks at a time. For compression damping, increase until expansion joints or sharp bumps are jarring, then back off one click. Wheel chatters over small bumps during braking or downhills. Too little rebound damping.
Rear end takes first bump in a series but is harsh on later bumps.
Damping is gone because of low nitrogen pressure, tired oil, or damaged internal components.