Instructions
Clarify the length of time that each volunteer is expected to commit. If possible, list practice dates and times on the sign up sheet so that people know what they are getting into. Avoid excessive practices. For a reenactment, you should require no more than two rehearsals for units and more for speaking parts as needed.
Match each volunteer with the best role for them. If a volunteer signs up to be infantry, and does not want a speaking part, do not assign them a speaking part.
Mention that food and drinks will be provided at all rehearsals and reenactments. You'd be surprised at the increased number of sign ups for events when food is involved.
Use the volunteer's time efficiently. Be ready to rehearse when the volunteers arrive and let them go a little early if possible.
Recognize and thank all the volunteers at every rehearsal and publicly at the reenactments. The recognition will make volunteers more likely to sign up again.