I am not anything more than an amateur historian, but my career has been 
in Information Technology so I have something to offer back to this 
great list.

CD's and DVD's are great for portable, short term storage. As has been 
discussed on this list, they are not true archival formats and should 
not be relied on as such. At the best, you'll get read errors as certain 
sectors break down which might render a few files unreadable. At the 
worst, the entire disc will be unreadable. Interestingly enough, even 
music CD's are prone to this problem - I have a few music discs that I 
purchased back in the early 1990s that are showing signs of CD rot.

Magnetic tapes are generally the accepted way of archiving data for long 
term use. A properly cared for tape will last somewhere between 30 to 
(they claim, obviously this has not been tested yet) 100 years. Tapes 
created in the 1970s are still readable today, provided that the actual 
hardware needed to read these tapes is still available and serviceable. 
Tape also has the advantage of being able to hold far, far more than 
even dual layer DVD's can. The native (uncompressed) format of LTO3 
tape, for example, can hold 400GB of data, and with compression hold 
double that. It's also more cost effective, being about $70 on the 
street, although the drive itself will cost far far more.

With the declining prices of hard drives these days, I am tempted to 
suggest getting several external hard disks and using them to archive 
files. I would make exact copies of the data on each disk, and keep them 
in a climate controlled environment. The most important consideration 
for the storage of magnetic media (hard disk/tape) is to keep the 
devices away from any electromagnetic fields.

You can also use a piece of software to create what is known as parity 
archives. Say you had a bunch of pictures that you wanted to store for 
long term use. You could archive them into chunks using WinRAR (RAR is 
open enough of a format that I would expect it to be readable far enough 
in the future) then use a program like QuickPAR to create parity 
archives. If some of those RAR archives get corrupted or damaged, you 
can use the PAR archives to restore them. It's a pretty ingenious system.

http://www.par2.net

Of course you would always want to do test reads/restores every so often 
to ensure the quality of the archive and to move the data to new formats 
if it looks like the old one is in danger of dying out.

I hope this was at least somewhat enlightening.

Yours,
Ben Ruset