As you say, lots and lots of hands-on. Pair the newbie with senior guys who love talking about all the weird stuff they've seen over the years. It's one thing to troubleshoot a device branch that's gone completely incommunicado after it tees from the mainline, but something else entirely to sniff out the intermittent ground fault in a remote display module. As they learn, have them spend some time at the office with the guys designing new systems so they get a grasp on not just what it looks like in the field, but why design choices are made. That way when they walk into an entirely new building on their own for the first time, they have a decent shot of knowing where to start looking.
