You know them -- they're the ones in the matching t-shirts herding the crowds into screening rooms, asking for your audience award ballots, blocking your way into that hot party.
They're also a great resource for promoting your film. As official reps of the film festival, volunteers get asked for film recommendations constantly. Make sure they know who you are and everything about your film. Make friends with them, give them promotional swag (buttons are always good), and just generally charm the socks off of them. Your attendance numbers will thank you.