It is important to set the expectations of testing before starting the UAT. The key messages that should be put across during the UAT kickoff are as follows:

• Finding bugs is the goal of performing UAT. If you find them in UAT, it's a great thing. Don't get frustrated because you've found issues, get stuck in testing.
• Focus on first verifying all the critical business scenarios before getting into exceptional scenarios that won't happen frequently. Follow the 80/20 rule to define focus. This is also a good time to remind everybody about the goals for the project.
• Review the reports from the previous testing and communicate any open areas:
  • Communicate the schedule for testing and retesting.
  • Cover the tools/processes to be used for logging bugs, triage, and communication after fixing the bugs.
  • Set the sign-off and exit criteria (communicate upfront that they need to sign off at the end of it). Set the client expectations with regard to the types of bugs that are expected to be fixed versus deferred. If this is not clear, you might struggle to obtain sign off with low-severity bugs open. Usually, severity 1 and 2 bugs must be closed for sign off, while severity 3 and 4 bugs can be postponed or handled post sign-off.