Conduct Open Applications for Jobs
Any role of responsibility for the organization should be acquired through some form of competitive application process based on merit and capacity. Hiring a friend may be convienient but it is also a form of corruption.
Publicize openings, what is needed from the applying party, and use an ad-hoc panel to conduct a fair hiring process, even for volunteer positions.
Once the person or group for the position is selected you will need to negotiate a detailed contract that specifies expectations and the extent and limits of authority and responsibility. Any future performance reviews can than be compared against documented contracts. Contracts should be continually referenced and updates negotiated as needed.
Recommended Resources: Finding and Hiring Good Employees