Offsite backup. It doesn't have to be elaborate, just leave a CD/DVD/chip in your desk drawer at work, or at your mom's place, or anywhere else you have access to that wouldn't be at risk in the same disaster.
Online storage. I use Dropbox. You don't get a ton of space with the free account (2 gb to start, an extra 250 mb for every friend you invite, up to 8 gb) but that's enough to store a few thousand ebooks. It's also handy for remote access to your stuff.
Here, use my sign-up link, and you and I will both get a 250 mb bonus:
Sign up for Dropbox here