Our church has a membership roll, so our first step is making sure that potentials are either members, or are in the membership process. (Incidentally, we find that, especially with today's younger attenders, church membership is not seen as a necessarily valuable thing, so getting them into membership is a hurdle that potentially doesn't need to be there. Anyone else?)
Second is the aptitude assessment. We ask volunteers to honestly assess their skills and passions. If a person is really passionate about media ministry but doesn't care to work with kids, then we are careful to make that distinction.
As the media guy, I onboard my volunteers by having them sit in every position during production over a series of events. They all get hands-on time with cameras, computer, lights, sound and our director's position to give them an idea of how the media operation works. From there, we try to encourage them to specialize in one or two positions while maintaining a working knowledge of how the rest of the positions operate. My righthand man and I let the other volunteers train one another while we oversee and maintain quality control (and fill in the gaps).
We have found that, letting the volunteers train new volunteers gives them all a sense of ownership over how services run, and that breeds more confidence in their own abilities to run services and problem solve.