From my employee communications training, I find that the most effective thank-you’s to volunteers are timely, include specifics, are tied to a larger endeavor/purpose, have a personal touch, and in some cases are public where appropriate.

Gary Chapman wrotea book called “The Five Love Languages” in which he describes various ways in which people express love and wish to have love expressed to them. The church I used to belong to (before moving to another city) was training its team leaders to use these principles to be better at expressing gratitude.