Read the News
The Arch Linux News is posted here:  You can subscribe to the rss feed by adding:  to your favorite feed reading software. You can also get news by subscribing to the Arch Announce mailing list: 
Pay special attention to news items with "manual intervention required" in their header. You can avoid a lot of trouble and embarrassment by reading the instructions in these news announcements and following them.
When you update, check pacman output for instructions related to updated packages
pacman -Qo to see if something owns them
Search for .pac* files (how to use locate, and suggest tools like meld and vimdiff)
Check for out of date or unmaintained aur packages on your system (sometimes these can cause problems when you update)
Check the size of /var and clear pacman's cache once in a while. A usefull tool to assist in this process is Cacheclean.
check disk (fstab options to check at boot)
search logs for errors (list scripts, tools to make this easier/more automated) look into errors when they come up, don't wait
link random libraries together to get a program to work
update once a year
copy - pasting commands into the terminal without at least reading the man page to see what you are actually doing to your system