There's no special way to back up your music library. Back it up the same way you back up the rest of your files. If you're burning to CDs or DVDs, Toast 7's disc spanning feature will make things a lot easier.
I do NOT recommend using iTunes to burn CDs as backups. It may seem like the natural route, but it doesn't maintain your directory structure or anything like that. I think it also renames your files (although that might only be if you burn an "mp3 CD" rather than a "data CD"). Basically, it's more trouble than it's worth.