Using DVD-R and DVD+R, sometimes DL (for expansive projects).
The entire session backup with all files (and usualy a text file documenting the session in brief) goes into a plastic document sleeve along with the session template/recall templates, PQ log, QC printout, copy of the invoice and a copy of the master (usually DDP on CD-R/DVD-R). Each session fits on one sleeve, I can get quite a few sleeves in a 3 ring binder, and I have a shelf full of 3 ring binders...
Still, it's saved me several times when a client shows up years later and says "remember that album you mastered for me a while back, well...." and they don't have the original mixes anymore.
DVD is a bit slower than HDD, but I usually have Toast running in the background making backups while I'm doing other things. All active projects (colour coded depending on which phase they're in, i.e. if they've been started, delivered, approved, backed up, etc) are on the main set of 500GB networked drives.
For those of you using HDD, do you regularly start them up to avoid having the bearings freeze up from lack of use? I read somewhere that the bearing lubricant can migrate/settle and the HDD can lock up on power up if not used for some time (i.e. a few years).
Thor