The approach to recruiting volunteers should be as careful as the process used for adding any new staff.
See A Legal Handbook for Nonprofit Corporation Volunteers (http://www.iciclesoftware.com/VLH7/index.html):
Volunteer screening and placement If, as some authorities suggest, a volunteer is a worker serving without consideration, then a nonprofit corporation utilizing volunteer services should put substantially the same amount of work into locating and placing volunteers that it would put into locating and placing paid employees. Volunteers frequently have a highly public role on behalf of a nonprofit corporation; they are the persons that individuals dealing with the nonprofit corporation see first and most often. You can manage several risks by effectively screening, evaluating and placing volunteers.