NOTE: These rules were learned empirically over many years in the Electrical and Computer Engineering Department at Ryerson. Every time we learned a new lesson, the rule was added to the "Book of Armaments".
Anything that can go wrong, will go wrong (If you fail to plan, plan to fail).
Never perform major hardware or software upgrades on a Friday; Wednesday is the best day (on Friday when things go wrong (see No. 0), you'll spend the weekend fixing them).
When pulling cable, always pull extra cable (10' extra typically!); in a small office, make sure the cable can reach every corner. Someone will inevitably want to re-arrange the location of all the computers in the room.
Any changes to the system should be immediately tested (new software installations, configuration changes, etc.).
Never delete an old version of software until you are completely sure that the new version works.
Learn from your mistakes (mistakes are inevitable; graceful recovery is essential!).
Failure is not an option.
For every action there is an opposite & equal reaction. Carefully consider the reaction to your actions (what you say & do; policies you implement).
Work smarter, not harder. Whenever you think: "There's got to be a better way to do this", it means that there is.
Always have backups.
Having backups of backups (stored offsite) is nice too.
Check your backups daily.
Only people with a key to the room are allowed in the data-centre. People without keys must be escorted at all times when in the data-centre.
Nullius in verba: do not believe any user's statement without proof.
Never close your last root shell.
The only reason to answer the phone when it rings is to make the caller happy. If you're unable to do this, don't answer the phone.
Do not brag about your network security publicly; you will be shamed publicly.
Do not announce publicly that you, "the sysadmin", will be going on vacation; your network will be compromised while you are the furthest distance from being able to do anything and you might as well not report back to work afterwards because you won't have a network to administer anymore.