In addition to documenting information such as system serial numbers and software license numbers, consider documenting your network drops.
Quick and easy to implement, documenting network drops can save a lot of time and effort in the event of an equipment failure on your network. Be sure to record which computers and devices are connected to particular ports on your patch panel and network switches. This can be done on paper or electronically, but no matter how you choose to document this, I would suggest posting a hard copy near the network equipment. This can be very useful in troubleshooting a bad switch or router.