My favorite (didn't actually know this):
Whatever technology you buy today will be obsolete soon, but you can avoid heartache by learning the cycles. New iPods come out every September. New digital cameras come out in February and October.
The most important (everybody knows this, but who actually acts on it?):
Come up with an automated backup system for your computer. There’s no misery quite like the sick feeling of having lost chunks of your life because you didn’t have a safety copy.
Hat tip: The Dynamistress