We use LaCie's SilverKeeper for backup. It's free. Once you establish a backup set between 2 drives, it checks through the data and synchronises between the source and the backup.
We'll backup at least once a day, on heavy tracking days, after working on every song.
When the project is finished, depending on size, we will either burn several different DVD sets, on different DVD brands, or we will dump it onto the clients drive. We provide drives when needed.
As my boss masters the majority of the projects we work on (he does some mixing and all mastering, I do all tracking), we burn DVDs of the mastering session and the final mix files.
We no longer keep any data, we had a couple of archive drives fail, so we are no longer interested in storing data.
Most of our clients are not interested in keeping the multi-track files. The ones that are will not pay for the studio time to ditch all the excess data and burn DVDs.
In projects of my own, I burn mutliple DVDs sets to different brands and hope. I should really buy an archive drive as well.
I'm interested in tape for long term archival, but can't see this being any more or less relaible really. Heard too many mixed storys.