If you work on a lot of music, you probably know the importance of backing your work up. Hard drives are notoriously unreliable as long term storage solutions, and there is nothing worse than the realization that you just lost a year's worth of work in the blink of an eye. Still, most musicians are pretty bad about this until it actually happens to them. (And it will sooner or later...)
Even if you're bad about backing up works in progress, chances are you've filled up your audio hard drive a time or two before and needed to purge material to make more room. Personally, I'm a believer in keeping everything, so any time I need to do this, it means I've got a date with a stack of DVD-Rs and Toast. (Believe me, I've had worse dates!)
I think it is very important, however, to think ahead when you are backing your projects up. You may need to restore a project for one reason or the other 5 years down the road. If that happens, you're going to be really happy if you document the contents of your disc thoroughly. This is also useful if you need to send a project to another musician, producer, etc. How you do this depends entirely on your own workflow, but here's what I do.
Name of Act
Name of Album (if any)
Name of Song in Quotations
Name of Producer/Engineer
Is the song a demo, pre-master, or master?
Backup Medium (CD-R, DVD-R, DVD-RW, Tape Drive, etc.)
Date of Back-up
What DAW format and version number is the project in?
If there are other proprietary file formats, what software and version number was used?
If the project is for a client, it's not a bad idea to include their contact info. Make sure to write this info on the storage medium itself too, as CD-R's and DVD's frequently get separated from their cases one way or the other.
One final thing to keep in mind is that occasionally DAW programs go under drastic changes in their native formats (such as when Logic Audio Platinum went from version 4 to 5), that may make your project unopenable in a future version. One option here is to keep every version of the program on your hard drive. I think the better solution, however, is to restore your old projects and convert them to the new format and then re-save them on a fresh DVD/Tape, etc. If you are really serious about keeping all of your old projects, it's not a bad idea to do this just to save it to new media every 5-10 years anyway. That's probably a bit anal if you store your backups well (in a cool, dark place), but no one ever regretted backing things up too much. The same can't be said for those who don't do it enough.