Before working with a consultant, ensure you have a firm plan of action prior to signing a contract. All parties should know upfront the scope of work to be evaluated. You must also ensure that confidentially is discussed before, during, and at the conclusion of the project.
In order for an external consultant to fully understand your business, they must have a complete picture – the good, the bad, the financial issues, etc. Do not hide negative aspects of your business, as the consultant will need full knowledge for their work to be successful. Give them the freedom to discover the underlying issues in your business, so that they can use their skills and knowledge to help your face its challenges.