I think it's helpful to spell out in detail what you're looking for people to do and the time commitment required. Also, make the time commitment realistic so one's not required to be there all day.
I thought NICA did this really well last year. I don't recall the race, but they outlined specific roles needed and time requirements. made it easy to determine how you could plug in.
I'm the PTA President for my son's elementary school and we rely heavily on volunteer labor. The above works really well and we use Sign Up Genius to publish the roles, times, etc and manage communication between volunteers and event leaders. We also advertise like crazy. We push hard on all of our comms channels so people know we need help.
We also partner with our districts middle and high school to recruit student volunteers. We have a
Key Club chapter which requires volunteer hours so they're typically eager to help. Look for something similar in your area. It's a good labor pool when matched with the right responsibilities.