Rule 1: Trust No One Completely
The terms and conditions of the service to be provided must be clearly stipulated in a contract. Never take your provider's word for it.
Ensure that the contract provisions at least cover these items:
Service availability hours, performance guarantees, and list of responsibilities
Support coverage hours, mode of support available (telephone, email, onsite, and so on), escalation procedures
Contract penalties, termination clauses, renewal and continuance terms
An important point to consider is what penalties are incurred by the vendor when the service is poor or unavailable. Definitely this penalty amount will be trivial compared to your business losses, but the higher the penalty stipulated, the higher the provider's motivation to make their service perfect.
The better providers will give rebates or free advanced subscriptions if their committed service levels are not met. Even better is a provider who ensures against business losses for service unavailability.