Keep in mind that environmental considerations are also important for storing backups, just as with negatives. CD-Rs and DVD-Rs can be prone to fungus, and should be stored in individual jewl-style cases (I like the thin ones) so nothing touches either surface. Also, CD-Rs are very fragile. Avoid scratching the top surface at all costs. If you write on them, write at the center, where the spindle goes. And store them in a low-humidity, relatively temperature-controlled environment. Oh, and keep two copies of each, in a separate location. Also, I've had bad luck with CD-RWs... I don't use them if I can avoid it, but YMMV.
Magnetic Media (discs, hard drives, tapes, etc) will all lose their magnetism over time, and environmental considerations are important for them as well. I don't think I've ever had a floppy disc last over a year (does anyone even use them anymore?) and my experience with DAT tapes indicates they don't last very long at all if they're used on a regular basis (IE for data retrieval at random intervals). The hard drives in my computer have been running for several years each, with no serious data loss that I've noticed... but they don't move. I don't know how external hard discs would hold up.
Solid-state electronic memory (like those USB memory sticks) are handy, but terribly unreliable. Plus, they don't hold enough to be useful, and the price is ridiculous.