Backups are absolutely essential, as we all know (some more to our dismay than others.)
I have multiple backups: I write (and save -- frequently!) on the hard drive of my machines at work and home, with the addition of a thumb drive that I use to take it back and forth. (I do "save as" onto the thumb drive when I'm done with a session.) Then I also print it out on paper (switching it all to single space, to save paper; with my other WIP I also convert it to a font size of 10; it's just backup; I never really have to read it; besides, I'm still pretty ok with tiny print.)
I've also thought about uploading it to the web somewhere. I do have a couple of websites I could "park" it on, but so far that seems like a little too much trouble. Perhaps I could also copy it into a blogger post that I never publish (just "save as draft") and then delete afterwards, but I'm not sure a post can hold that many words. Besides, even though I've had relatively little trouble with it, I've read that Blogger can be unreliable.
What other backup methods do you use?