Like Mike, I use hard drives. I've found tapes too pricey and slow, DVD-R's too unstable, and I still feel weird about using nothing but HD's, but it's the best price per megabyte going. My system is like this:
- Production RAID: This is a 2-disk mirrored RAID that holds all my working files. I use a fast single HD for a scratch disk. I don't do enough work that the write speed of this RAID gets in the way too much.
- Backup RAID: This is another 2-disk mirrored RAID that does nothing but clone the Production RAID, at the end of every day.
- Offsite RAID: Once a month I bring yet another 2-disk RAID in from another location, clone the Production RAID, and take it back offsite. This is usually on a Friday, and I return it to its offsite location on Monday.
- I also do daily clones of my app/system folder internal HD to a single-disk bootable clone.
Rackmounting your HD's makes this a lot less messy than it sounds. :-)
When one gets low on space, I upgrade all 6 disks at once, keep the offsite RAIDs as permanent archives, and rotate the other drives into service as field HD's for recording or to replace my scratch disks, or to just serve as emergency backups...this major upgrade cycle only happens once every 3 years or so.
As long as my house doesn't explode on a weekend, this has saved my bacon a number of times. It could still be better, and I'm looking into high-capacity cloud backups this year as another tier of safety.