With the current situation, you are looking at copying your data from an obsolete media to a more recent one every five to ten years. This situation is unlikely to change any time soon. The important stuff you may wish to have on more than one media type, anyway...
The safest bet if you can spend the extra buck is, to my knowledge, DVD-RAM, which has excellent durability and an effective built-in error correction scheme. The disavantage is that you need a separate drive (or better, two), and due to the error correction and write-verification, it's a little slow.
Other DVD-/+R media are cheaper, but a little more prone to data corruption. CD-R has an inferior error correction scheme, but is definitely a cheap solution. Not very handy if you have a lot of data to back up, though.
Hard disks, or any other magnetic media such as tapes, have the disadvantage that the magnetic charge fades over time. For hard disks, this might very well be 10 years or more, but I think you are better off with optical media, which do not show this problem at all.
Another problem is physical storage - I'm not even daring to offer a viable solution there!!
Hope this will kick it off
Ralf