It's a hard problem, but I've had some success with a few approaches. One is pulling the team into a conference room and giving a deep-dive presentation on some

It's a hard problem, but I've had some success with a few approaches. One is pulling the team into a conference room and giving a deep-dive presentation on some aspect of our system. I made a point of doing it via a web conference with recording enabled so someone could always go back and watch/listen to parts in a pinch. Another is calling an impromptu huddle when I'm about to work on something that nobody else feels prepared to handle on their own. When some crazy bug crops up that I'm about to troubleshoot, I'll often ask a few other team members to shoulder surf while I talk through the approach I'm using to run the bug down. It's a great time to ask the group what they think we should do to test their knowledge.