"For want of a nail" is the beginning line of a well-known children's poem (sometimes attributed to Benjamin Franklin) that explores the consequences of a small failure—neglecting to carry a spare horseshoe nail, which eventually leads to the downfall of a kingdom. Overlooking little things can also make the life of an Exchange administrator difficult.
Consider something so basic that you've probably never given it a second thought: the mass of cables and wires attached to your servers. Do you know what all those cables and wires do? Do you have spares on hand? Does everyone who might need to work on the server know which connections go where and what each connection is for? A friend of mine recently spent an entire Saturday trying to figure out why his Exchange services wouldn't fail over from one cluster node to another after routine hardware maintenance. The culprit turned out to be an improperly reconnected Storage Area Network (SAN) cable that kept the second node from reserving the cluster's disks. If he'd only double-checked the cable connections before doing anything else, he might have had a more enjoyable weekend.
What about passwords? I'm not talking about your account passwords—no doubt you know those (and you'd better not have them written on a sticky note stuck to the side of your monitor, either!). Think about the other passwords: for your routers, switches, SAN equipment, and so on. Chances are excellent that you haven't thought about those passwords in a while, which greatly increases the odds that you won't be able to remember them when you need them. Take a moment to list all the password-protected devices you have, record the passwords, then store the password list in a secure place. DON'T use the same passwords for all devices, and don't use your Windows account password for any other device.
Do you have manuals for all your hardware? Heaven knows, I don't. That's OK, as long as your hardware vendor makes documentation available online. But you might want to take time now to make sure that documentation for your prized 1993-vintage DHCP server, for example, is still available somewhere. Just in case, you understand.
If you have service contracts, do you keep a copy of all the relevant paperwork in your machine room? If you do, you don't have to waste time hunting for the documents—especially if you're at a remote site—when you call the vendor for support. For critical pieces of hardware, keeping a list of service ID numbers and contact information on your handheld is a good idea.
Product license keys round out my list of small-but-critical things you need to be able to find in a hurry. These obnoxious critters can spell the difference between a successful installation or recovery and a frenzied hunt for an orange sticker with an optical character recognition (OCR)-printed string of gibberish. For products that have license keys, I use a Sharpie pen to write the key on the product (e.g., on the CD-ROM). Then, as long as I don't lose the CD-ROM itself (hey, another thing to keep track of!), I'm in good shape.
If my wife were writing this, she would digress into a lengthy set of instructions for keeping your office or server room so neat that you could always find what you're looking for and you wouldn't need to remember all this stuff. I'd write more, but I think she's calling me to go clean out the garage.