Murphy's Law: You will not remember that brilliant find + grep + sed + ps + expect hack you pulled off 47 days ago. 5 systems is about to go down in flame unless you recall it soon.
Logging: What some thinking newbies learn to do after 6 months of un*x use. If you discover a new way of solving an infrequently recurring problem, add a brief write-up to a single text file owned by you (not root--nothing sucks more than snarfing and barfing 50,000 unix commands into a root shell accidentally). Before saving the file, add a few keywords you can use to search out later (make them all caps. Then use the file viewer less and /keyword to retrieve the write up later. Aside from keywording and a brief write-up, feel free to include a healthy amount of cut and pasting of the exceptions and quirks you'll never remember and will definitely run across later.