I've got too much stuff to make it practical to back it all up, so currently I don't back up anything. Oops.
Actually, that's not quite true. I do at least have resilliency against single drive failures - two 2TB Drobos and two 400GB RAID1 arrays. I've recently had both of the disks on one of the RAID1 arrays go offline, but I broke the array and both drives are still readable so I didn't lose anything (I also had two days warning between the two failures and took the opportunity to copy all the data to one of the Drobos).
I need to re-organise all my stuff (about 6TB worth). A lot is old rubbish that I should dump. My plan will be to organise stuff into five priority levels:
1. Important documents, valued photos, etc. Sync offsite - Dropbox or similar.
2. Documents, work files, photos, software I'd really rather not lose - keep on a Drobo and a second disk (TBD). Archive to DVD where possible.
3. Work files. The most important stuff is backed up already - version control and fault tolerant hardware. The rest I need to decide what's best. A lot can go into category 5.
4. Media collection. Far too big to back up, so I won't. It'd be an irritation if I lost it but not the end of the world.
5. Rubbish. Bin it.
The problem is finding the time to do it - so far I've just ended up firefighting. If there's a problem (be that a disk error or full disk), I've done whatever was needed to fix it. No significant data loss to date but I'd best watch my back, or get bitten.