As soon as you complete an initiative you need to recognize volunteers’ contribution to it as much as you can.
Social recognition and praise are positive feedback. You need to regularly tell the community about successful initiatives and their authors though all media channels you have. The more volunteers feel that their contribution is recognized fairly the more likely they will propose and participate in new initiatives. Visible examples of successful initiatives and public recognition of the contributors motivate other users to become volunteers themselves.
This is a fragment of a draft of the book “Lessons Learned While Working On Stack Overflow”. Read the full book on kindle or the paperback version.