I don't think this has been covered elsewhere, but I've found a post here that describes how to get Time Machine to back up to a network share. This might be useful for people who don't want to get a USB drive just for TM when they already have an NAS.

I had to do both the Terminal command on page one and the Disk Utility instructions on page two. I'm testing it now, it seems to be happily backing up the whole machine to my LinkStation.