Interview each volunteer as if they are seeking a paying job and hold them to high standards. Be on the lookout for unusual opportunities. Here in the U.S., we at Charlotte's Web are getting ready to develop some related businesses to generate money for our nonprofit corporation. We hadn't thought of that originally, but it will become increasingly an important part of our support. Hundreds of groups have already tried what you are doing and most will be only too happy to share with you.
Overwork only hurts the movement in the long run.