we have had pretty good luck with volunteers by sending out forms that parents can return with their preferred times. so if our carnival runs 6-8 pm, there are half hour time slots through the night, and people can pick the most convenient time for their family.
there is a master volunteer table ( on an excel spreadsheet )kept on the pto bulletin board. it's updated daily with emails/ notes sent back from parents.
our food stand is always the preferred location for volunteers, for some reason-- we cut down on the number we needed by expanding those time slots to 45 minutes starting 15 minutes before the event.
now, sometimes people are slow to sign up-- I always send out a mini table of current volunteers reproduced in our friday notes or newsletter. The blank spots speak for themselves, and people can fill their name in right on the table and send it back through backpack mail.
this year our school is teaming up with some local high school community service teams to get more volunteers as well.