How I did it: (Mostly copied and pasted from my entry from "Volunteer for London Fringe 2008" goal)
I volunteered for the London Fringe Festival last year, after finding out about it from friends who volunteer at the same theatre I did in the summer. They were volunteering at Fringe as well and told me where to go to sign up. I checked out the website, and just went to the Fringe headquarters and asked if they needed any help!
Lessons & tips: Just ask! Get your courage up, email/call/visit the headquarters, and ask if they need help. Most places seem to be great with this! And non-profit organizations generally need all the help they can get!