I've heard many reasons why a potential volunteer doesn't call an agency. Many of them simply never think to ask if you have short-term assignments available. To do this you need to identify tasks that can be done on a one-time or periodic basis. Be as specific as possible. Five people are needed from 2-4 p.m. on Friday afternoon, October 4th, to decorate for the annual fundraising dinner.
Help is needed to blow up balloons, hang streamers and place party favors on tables.
Three people are needed from 9-10 a.m. the first Monday of each month to assist the office manager with the newsletter mailing. This way, the volunteer is in complete control of the amount of time they give you and can take on as many or as few tasks as they feel they can handle. By giving people the option of stepping into a project for a specific amount of time, you create a win-win situation. They feel good about being able to help within their limited time constraints, and you are able to get your work done with the help of someone who otherwise would not be helping at all.