You need to buy an external hard drive for no purpose other than to save your items once every few months, and then unplug the drive and let it sit there. It's cheap - less than $100.
Consider this: How many computers have you had in the last 10 years? I've had, off the top of my head, 10, including desktops, laptops, work machines, and personal ones. Each one had data that appeared nowhere else, and most of those machines were replaced because they died - making it hard or, occassionally, impossible to get the data from the drive. It isn't just me. I've helped a lot of you out there with the same.
Now consider what is on those drives. Here's what I would miss:
* All the archives of design projects from the last 10 years.
* Archives of my law school notes (I still refer to them to brush up on basic principles of law once in a blue moon).
* Archives of personal docs ranging from taxes to the letter I sent my future in-laws regarding my engagement.
* All my photos from trips, days at the park, my wedding, my honeymoon, and so on.
* My Music.
These things are stored only on my hard drive. I don't carry an iPod with all my songs. I don't have DVD backups of these items and the libraries have gotten so vast that they would take a spool of CDs or a short stack of DVDs. Oh, and one backup isn't enough. I have one backup. A week after I got a cold backup drive, I turned on my machine and the regular external drive just wouldn't boot up. It turned out to be a one-time glitch, but it made my blood run cold -- and I already had a backup.
We don't fully appreciate how data-dependent most of us have already become. This is gentle wake-up call. Need suggestions? Shoot me an email.
[Edit: As the commenters have correctly pointed out, if you have really valuable data - like photos of your little kids - you should back that up on a third drive and drop it off with a friend or relative. That protects your data against theft and fire/flood/Armageddon.]