Probably Networking 101 but easy to forget: Always follow the ISO layer model when troubleshooting. I've had (and seen) more than one instance where troubleshooting started at some higher layer, only to find a while later that it was a bad cable. This applies to non-network-related troubleshooting as well.
When uploading IOS images to a switch for IOS upgrade, always run the MD5 checksum once it's on the new switch and compare it to the MD5 of the original file to ensure that the image hasn't been corrupted. I've seen that happen more times than it should, and the last thing you want is to reboot a switch during a short change window and find yourself at a ROMMON> prompt.
