The answer is to maintain multiple copies. Corporate backup strategies almost always also require at least one of those copies be stored offsite. That way, if a tornado wipes out your building, you can recover your data from the remote location. For most people, this would be done by taking a copy to a trusted friend or family member's house. But consider the recent flooding in Australia, where large areas are underwater, or the aftermath of Hurricane Katrina in New Orleans. Is a copy at another location in the same city sufficient? You will have to decide for yourself.
|USB flash drives|
|External hard drive and USB cable|
|Add an extra hard drive as a mirror copy|
One last method is what the tech community is calling Cloud Storage. That means having some storage space somewhere on the Internet where you can upload and store (and maybe even share) your data. This could be as simple as a Dropbox-style service, or maybe uploading your tree and data to Ancestry.com or other service designed for genealogy. The benefits are that the storage is by nature offsite, and almost certainly redundant in that those services maintain several failure mitigation controls. A drawback is that once your data is out there, you don't have full control over it. You never know for sure who can access it, and for what purposes.
I would recommend a combination of these methods. I personally use the mirrored drives approach, and copy my data to DVD as well. I could do better at the offsite storage part of the equation. (See, even techs don't always follow their own advice!)
What method(s) are you using for data backup? Let me know in the comments below!