DEAR MISS MANNERS: I have been asked for a good way to remind our senior volunteers of their commitments to work a couple of hours a week at the public library bookshelf, where we sell used books and donate the money back to the library. I have just assumed the bookshelf management job, and have never had responsibility over volunteer workers.
GENTLE READER: Managing volunteers requires both firmness and tact. Firmness, in treating the duties enough like a normal job that volunteers comply. And tact, in that you must be more patient and understanding of partial or noncompliance -- and more effusive in expressing gratitude for work done -- than a boss who is signing paychecks.
In the case you describe, Miss Manners recommends something as simple and direct as a sign-up sheet, perhaps followed by an automatic reminder.