There are two basic approaches you can take that don't involve additional hardware.
- You can partition the drive into a "data" volume and a "backup" volume. A program like iPartition can be used to do this "live" with data in place, or you can use Disk Utility if you can (temporarily) move the files off the drive.
- You can use a sparse image, as discussed in
the FAQ.
Either will work, but the former is preferred because it's faster and will allow you to boot from the backup if (when) disaster strikes.