A good support plan makes it obvious how requests are submitted, prioritized and resolved.
Define standard requests such as content edits and bug fixes separately from critical incidents such as downtime or security events.
Set realistic response targets rather than promising impossible instant resolution. If you offer 24/7 support, document what 24/7 means: monitoring, triage, human response, or all three.
Monthly reports should summarize completed work, incidents, security findings, backups and recommendations so the client can see the value of the service.