Ask them to do a job that has a definite beginning and end. Don't require too much from members. If they know the job is small and won't require a lot of time and effort, they might consider helping. Give members a range of jobs from which to choose. Ask members to do things they do well.
Volunteers are willing to do things they know they can handle.
Tell each person how their job fits in with the campaign. Volunteers want to understand how the whole campaign is going to work and how their part will contribute to it. Encourage questions if members don't understand something. Keep volunteers accountable. Check back to see if and how the member is doing the assignment.