Beyond the excellent information already given, I would add that, when you approach the GA, don't simply ask about volunteers, but rather point out that you have some flexibility so if they need to re-route anyone, you'd be happy to volunteer.
Yeah, it's saying the same thing as "Are you looking for volunteers?" but in some cases I've heard people asking that in a manner that seems more opportunistic and sometimes close to demanding than trying to help out.
It's educational to hang around around the counter and listen to what goes on. Just imagine it's you behind the counter, listening to some of those stories.
