my strategy is never to set all my money to one horse only...
First of all one important backup is the backup from your NAS4Free config, do this first.
Generally I do not trust CD/DVD for 100%, but it is better to have one more backup on CD/DVD than no backup.
I backup my personal data on DVD-RAM, this medium is not well known at home usage, but perfect for backup. Formated with UDF it is designed to survive 100.000 writes, long term lifetime 30 years and each written byte is verified while write, when it fails this sector is marked as broken, similar to a harddisk. Furthermore a mounted DVD-RAM can used like a regular harddisk with read/write by filebrowser, no need for a burning program. Many DVD burners are able to write DVD-RAM. Disadvantage is that not all operating system can use DVD-RAM with UDF, on Debian/GNU Linux it runs out of the box
Some USB pendrives are used for backup from personal data likewise. On a 16GB Transcend stick all the data from our family fits, only audio, video and the large photo collection needs a separate location.
I use a second NAS and some usb harddisks for storage, furthermore some different tape drives, but only because I got them for free.
When you use harddisks you must choose a file system and decide where you plug in the drives. All of my Desktops and Laptops are equipped with Linux, so I use ext3 and plug in the drives to my clients, data is transfered by network. I do not want to plugin the drives to my NAS because I made some bad experience with the USB implementation from FreeBSD, furthermore my NAS machines are located in the baseground. From time to time I
BTW: Please note that data can be affected by silent corruption, I use md5deep (available for Linux/BSD/Windows) to calculate checksums for my data, write them to file and store it together with the data. When you later verify the data against the checksumfile you can be sure data is not corrupted.
A second tip, always store one backup media outside your house, the best backup does not work when it gets stolen, house is burning and the rest from the drives is killed by water...