In a biblical church there are only ministers. We should treat those who serve without pay with the same respect as those who are paid for their services. Always keep the vision of ministry before your people. Communicate the importance of their ministries. When you recruit, emphasize the eternal significance of ministering in Jesus' name.
Vision motivates people.
Guilt and pressure discourage them. Help people see that they are investing for eternity and that there is no greater cause than the kingdom of God.