Another item, and this is experience I paid in blood to have: I have had every single backup medium fail on me. I've had CDs go bad, tapes go bad, hard disks fail, hard disk RAID controllers fail, trashing multiple disks, cloud storage places zeroing out my data due to a client glitch, and other storage places "forgetting" I had an account there.
When backing data up, have at least two different mediums. On my Mac, I use Time Machine to an encrypted external drive. I also use a tool called rsync to copy just the changed files from my documents folder to a mounted TrueCrypt container stored on Dropbox. This way, my documents are saved twice. One on the Time Machine drive, one on Dropbox. This way, if a thief steals my laptop and external drive, due to the encryption, it becomes "just" a hardware theft, as my data is protected, and I can just access the stuff stored on Dropbox via another machine. If my Dropbox account gets overwritten, I still have my original copy and my Time Machine copy. Finally, after tax season, I copy all my documents to DVDs and store those somewhere secure. I've never had to go to the DVDs (which are intended to be a last resort), but it is there if needed, and once a DVD is finalized, no malware on a computer can alter the data present.