Yes, we also get pretty specific, and ask parents to work 1/2 hour shifts. I encourage whomever is the volunteer coordinator for an event to make sure volunteers don't have to stay longer than the half hour. Sometimes people forget to relieve someone and then that parent doesn't want to volunteer again because they're afraid they'll be "stuck."
After two years of doing this, we always seem to have a lot of volunteers for events. We still have trouble getting people to step up and be leaders, though.
We email and post needs on PTO Facebook page. I try to be as specific as I can. For example , We need 3 volunteers to work 1 hour between 1-4pm on May 10th, as cashier for the Raffle Ticket Station during Spring Fair. Etc. I try to list as many jobs as I can. Sometimes people are more apt to volunteer if they know exactly what will be required of them.
We have many articles on the site that provide tips for reaching out to parents. It's an ongoing process that is helped along when parent groups create a welcoming and fun atmosphere -- that helps build a strong school community that parents will want to be a part of.
Also, many groups have had success with our 2-Hour Power Pledge -- Ask parents to pitch in for just 2 hours (for the year) and you may find many are comfortable with such a manageable request. (And some will enjoy volunteering so much that they will end up doing more!) Here's the link: www.ptotoday.com/2hourpower/