I had a router / firewall go down once when I was out of town and since I'm the only one that knows about the servers it was a major pain talking through the work-around on the phone. I couldn't get into my network and the outbound internet was down so the VoIP phones we have didn't work. The only way was via cell phone which, of course, had no service in the server room. We ended up playing the old-school game of "telephone" with one person yelling instructions to the next then yelling them into the server room. It took TWO HOURS just to bypass the router as was extremely frustrating. I've since made a complete network architecture with Visio and listed all the services provided from each machine. I also hid a paper list of all the passwords to the servers. I then called and interviewed several third-party tech companies in town until I found one familiar with pretty much everything we have. If something goes down again, I'll call them, point them to my password list, and cross my fingers that they can fix it without costing a ton of money.
I don't have a "bus plan" yet. It's such a huge challenge to teach someone all the SEO stuff I do for clients. I think I'll look into a transfer of ownership / trust thing in the future.