I just did this, using this process on OS X:

Create a DVD-sized disk image:

1. Open Disk Utility
2. Click New Image
3. Enter a file name and save it to a location where you have enough room for 4.7 GB
4. This is important name the volume something like backup or whatever you want, but you'll need to name all your subsequent disk image volumes for this backu the same name. Keep it simple and just name it backup for each.
5. Click create.

Run tar:

1. Open Terminal and run this command (this assumes you're backing up your Pictures directory. Change last element to your desired directory name.):

   $ tar -cvM --file='/Volumes/backup/backup.tar' 'Pictures/'

This command will create a muli-volume tar archive that will stop when you're backup volume is full.

1. When your backup volume is full, tar will tell you to prepare the next volume. Go create another disk image, using the steps above and name its volume the same as your first image.

That's it! I just did this to backup my files.

Burn the DVDs

After you completed making as many disk images as you needed and filling them with tarballs, simply burn them through Disk Image:

1. Open Disk Image
2. Click the Burn button
3. Select your disk image
4. Burn it
5. Repeat untill all your images are burned

Subsequent tar backups since your first full-dump:

At some point in the future you'll want to do this again. Instead of another full dump, as they call it in the tar world, do one that only writes files modified since your last dump. (tar does have an incremental archive feature, but it doesn't support files modified by the Finder, which is nearly every file you'll ever work with.)

1. Open terminal and run this command:

   $ tar -cvM --newer-mtime '2008-01-20 00:00:00' --file='/Volumes/backup/backup2.tar' 'Pictures/'

This command only adds files to the archive that have been modified since the date and time specified.