Well, you can either: a) get an external, or second internal HDD and just zip/rar the files (limited amount of space), b) burn them to DVD's (could get kinda expensive after a while), or c) get a complete HDD back-up program (like Norton Ghost or somethin), and save them in an alternate location.
I guess it depends on if you want access to the previous saved files later on. Those are some ways that I can think of, though. I'm sure that other members can think of other ways, as well.