Describe the role honestly, including the dull parts
People who arrive expecting something else leave quickly.
Volunteer recruitment tends to describe the mission rather than the work. The material talks about the difference you will make, and the actual role involves three hours of sorting on a Tuesday. Both are true and only one is what the person will experience.
The mismatch shows up in early attrition. Somebody who came to help people and spends their shift on data entry concludes the organisation misrepresented it, and they are half right. Describing the actual tasks, the actual hours and the actual proportion of routine work loses some applicants and keeps the ones who will stay.
It is also worth being specific about commitment. A vague suggestion that any help is welcome produces people who attend twice, because nothing was ever established. A stated expectation, even a modest one, is easier to meet and easier to decline honestly.
The other element frequently omitted is what the role does not involve. People arrive with an idea of what they will be trusted with, and saying plainly that a task belongs to trained staff avoids a disappointment that would otherwise be discovered in a difficult moment.
There is a specific recruitment message that works better than an appeal, which is naming the gap. Saying that Tuesday mornings are uncovered and what happens when they are is far more effective than a general request for volunteers, because it gives somebody a concrete thing they could solve rather than an open-ended commitment.