Keeping it clean
If you keep your PC running for long periods of time, it's likely to collect dust. Dust collecting on fan blades, heat sinks, and other internal parts eventually results in temperature increases. The PC gets hotter, becomes less efficient, and, in the worst cases, becomes unstable.
The better PC cases have dust filters, but those can eventually get clogged, which restricts airflow andyou got itincreases internal heat. For this reason, I pull out my main desktop systems every couple of months, open up the cases, and clean them all out with compressed air. I usually look for cans of actual compressed air, which are free of CFCs, HCFCs, and flammable gas.
Sometimes you need to snap a quick photo of a work-in-progress. Any camera will do, including your cell phone camera. The purpose isn't to record your fine, PC-building handiwork for a scrapbook, but to capture a moment in time so you can see exactly how small switches are set, and where cables are attached. In essence, you take a photo before you disassemble something so you'll be better prepared to connect everything back together in the proper configuration.
Sure, you can label all your cables, but a quick photo will help you keep track of just which wire attaches to which connector. Indeed, when I build systems for friends and relatives, I'll often give them a printout of all the connections on the back of the new system. Being the "family tech guy," I receive fewer support calls this way after I've closed their computers and gone home.
If you're routing wires and cables inside a PC case, you've probably needed to tie them down. One pro tip: Don't use zip ties. I loathe zip ties. If a system has wiring tied down with zip ties, and you later need to reroute a few wires or install something new, you need to cut off the ties. So instead, I use ordinary twist ties most of the time. If I need something a little more robust, I can use a plastic buckle tie or a variety of Velcro-equipped straps.
You can pick up Velcro straps online, at hobby shops, or local electronics stores. And they're tremendously useful for chores unrelated to PC assembly. I use them to organize bundles of Ethernet cables, the mess of cables in the back of my home theater rack, and in a variety of other scenarios. The possibilities are endless.
I've used IR thermometers to measure the temperatures of power supplies, graphics cards, and interior case locations that may be thermally sensitive hotspots. It's really a wonderful tool. You aim the thermometer's laser at a potential hotspot, and if you find your case is trapping heat in that area, you can add a case fan, or simply re-route some wiring. IR thermometers have become less expensive over time. The Kintrex 421 shown here can be found for under $50.
External SATA dock
If you want to move an existing system build (OS, applications, docs, settings, everything) to a new physical PC you're building, you can always install the old drive alongside the new one and copy over an image. However, it may be easier to attach an external USB drive dock. These come in a variety of connection types, and some will hold multiple drives. I happen to use a Plugable USB 3.0 single drive dock, which suits my needs. A USB dock is also useful for making quick and dirty backups, or when you need to move really large amounts of data via sneakernet. They're less useful, I've noticed, for SSDs: Trying to clone a drive from a system to an SSD in the dock seems to take roughly a galactic lifetime or two.
PCs have become more power efficient, but I've found that simple power meters are still useful to keep around. For example, with the help of a power meter, I once noticed a particular PC's average power consumption increasing substantially. The problem turned out to be dust clogging up the power supply fan. The fan couldn't cool the power supply properly, and as any power supply gets hotter, it loses efficiency and consumes more power. No, I wasn't worried about using extra electricityI was worried about frying my power supply by over-taxing it.
A power meter is also a useful tool for measuring any appliance or other piece of electronics. When we remodeled our kitchen, I ended up with two extra refrigerators, but only needed one for our garage fridge. I tested each extra fridge with the Watts Up Pro, and kept the more efficient model. The USB-equipped Watts Up Pro I use also allows you to collect data over time, and analyze it later.