When you need workers, ask for them personally. Don't rely on flyers or letters to do your recruiting job. On Friendship - Have someone they know do the asking.
People respond more positively to someone they know.
If a person agrees to help, welcome the new volunteer. Introduce her or him to the other volunteers. Establish the feeling of belonging and being needed. People don't like to take risks. New volunteers can often be induced to take on more jobs if they first have a few successes to look back on.