Speaking of upgrading to Yosemite, Ars Technica details how to create a USB installer for Yosemite. I usually like to do a clean install of a new OS and installing via USB is a great way to upgrade on your computer(s).
I also like to keep a USB installer of the previous OS and of course I have a bootable copy of my computer (backed up with SuperDuper) in case anything goes wrong.