There isn't much beyond what they did, assuming you don't have another datacenter to preemptively switch load to. But what you don't do is tell your customers it will be all good. You tell them what you've done and warn them they may be downtime so they can plan accordingly.
You might start by moving infrastructure out of the hurricane. Off the east coast would be nice, but at the very least outside of a mandatory evacuation zone [1].
Alternatively, you could design some type of system that would allow you to fail over to a geographically redundant datacenter. Joel claimed in 2007 [2] that they had such a system, and touted it as a selling point of the reliability of the hosted service. What has happened to it is probably only something that a Fog Creek engineer can tell you.
