I regularly take backup of my important data. Though I have my USB HDD for this purpose, I also keep some data on CD/DVD which is not meant to change over a period of time.
One thing that I always have problem with is deciding the right files (sizes) so that the blank media gets utilized to its fullest capacity and for this I use the software Burn to The Brim.
The software is very easy to use. You keep on dragging files in the interface. When the media is full or has exceeded is capacity , this software will automatically suggest the best combinations in which the files can be written to CD/DVD so that maximum blank space can be utilized. Just give it a try and let me know how you find it.