I avoid system admin as much as I can these days, so hopefully you'll never need to remember these tips either. If you do, here they are!
PostgreSQLLogging - How to log every query your PostgreSQL server processes.
MuttMaildirCaching - Debian mutt contains a Maildir header caching patch. It's worth turning on! Though maybe not! Some other mutt tips too.
ModemInitStrings - Modem configuration strings that set up modems for dial-in.
SubversionSetup - How to start a new repository for a new project.
IMAPSyntax - Commands for testing an IMAP server.
Sendmail Maildrop SpamAssassin - Some notes on setting up a small mail server.