The SLA should include components in two areas: services and management.
Service elements include specifics of services provided (and what’s excluded, if there’s room for doubt), conditions of service availability, standards such as time window for each level of service (prime time and non-prime time may have different service levels, for example), responsibilities of each party, escalation procedures, and cost/service tradeoffs.
Consider these SLA Metrics.
Management elements should include definitions of measurement standards and methods, reporting processes, contents and frequency, a dispute resolution process, an indemnification clause protecting the customer from third-party litigation resulting from service level breaches (this should already be covered in the contract, however), and a mechanism for updating the agreement as required.
This last item is critical; service requirements and vendor capabilities change, so there must be a way to make sure the SLA is kept up-to-date.