Make the purpose explicit
Explain the problem, the people affected and the business outcome the project is intended to support. Keep the purpose visible as decisions about scope and priorities arise.
Define the boundaries
Document what is included, what is outside the current scope and which assumptions need to be checked. A useful brief leaves less room for conflicting expectations.
Agree how decisions happen
Identify the sponsor, delivery responsibilities and decision points. Clarify how changes, dependencies and unresolved issues will be handled throughout the work.