Exercise. Stretching/strengthening this joint and its ligaments is the best options currently. However if it still persists you may need to be seen by a spine specialist. In our experience, the si joint is typically not the first problem that needs to be addressed. Often the facet joints and herniated disks tend to be a primary issue first that can help this pain issue.
Depends if it is. From your back or from a piriformis spasm. If from piriformis spasm, stretch. If from your back, consider physical therapy first. If that does not work, epidural, translaminar or transforaminal injections can help. Make sure you go to someone who can diagnose it correctly, as si joint pain can also present that way.