Document Document Document!!
So I spent all morning and part of last week documenting all of the connections in the server room. I've done this many times before, and it's my least favorite part of my job. Oh, don't get me wrong, I don't mind generating documentation! Realistically, it's a big part of being in IT. What I do loathe, however, is when the people around you change things (repeatedly) and don't update the documentation. So, the lesson for today is to get everyone on-board with some kind of change management scheme, starting with your manager for enforcement purposes. It doesn't have to be at the level of ITIL, but everyone needs to know that it's their responsibility to update the documentation when changes are made.
For our servers, we use SYDI to automatically document them on a weekly basis. When we fix an issue, we record the resolution in Spiceworks, our helpdesk software, so that we can find out what we did to fix an issue if it recurs. We run a ping sweep over an entire network on a monthly basis and check that against our Internal Network Address database (an excel spreadsheet) to see if our free IP addresses are actually unused. We have some snazzy Visio diagrams of network paths, server rack population, and power cable routing. The only part of this that's hard to do is mapping out what devices are mapped to which switch port. You can look into the switch's management interface and find out which MAC addresses are on that port, but that doesn't mean it's the only device (like if the switch is connected to another switch). Also, it's not easy to identify iSCSI HBAs or other odd devices like IP cameras and Virtual Tape libraries in this manner.
So, it's all down to tracing cables, unless you can get your team to write down what they did whenever they plug/unplug a network cable.