For home use, an external hard drive is probably your best bet. Its not going to protect you if your place burns down, but it will guard you against your computer frying (I have no idea if fire-proof safes are good enough to protect a hard drive in the event of a fire).

If you need a what-if-my-house-burns-down level of protection, then its best to find someplace to offload your data. I had a friend who was trying to write something that would encrypt and back data up to something like Amazon's S3, so that might be something you could look into if you need that level of protection.

The only sure-fire way of protecting your data is to back up to tape (or some other media), truck it to some off-site storage facility, and then regularly take your backups and restore them just to confirm that its still working. But at that point you're putting in full-time-sysadmin levels of effort.

Personally, I have a Linux filesharing server that I regularly back up to an external disk. This is good enough for most day-to-day scenarios, and I have very little data that would be devastating to lose to a fire or natural disaster (about the closest things are tax returns and immigration paperwork).

Changing media formats or filesystems is not a problem as you must make copies on a new medium every now and then anyway, if you care about your data. Most media, in particular the optical ones, live not nearly as long as the expectancy/advertising goes. In any case, even if they may live that long, they're not as reliable as they should be.

When CD came out, they said "100 years", which was later corrected to "roughly 20 years". I own CDs from 1984 which are entirely good as of today, but some CDs I bought in 2005 have developed audible defects since then.
CDROM was attributed "at least 20 years" when it came out shortly thereafter, and this was soon changed to "probably up to 20 years". In reality, you get dropouts anywhere between 2 and 20 years.
Similarly, DVD+R was said to last "100-200 years" while DVD-R was said to last "25 or more years", and as a matter of fact, my DVD-Rs rarely survive longer than 3 years before they begin to show some problems. They're not unreadable at that time, but the point is they do have errors, and you just can't trust a medium that has errors.
You first notice that when it takes much longer than usual for the disc to pop up a drive icon or showing a directory. If your drive is seeking around for 15 seconds, the odds are good that the medium contains enough randomly appearing errors which exceed the error correction's capabilities (4000 bits of data), so the drive is re-reading a couple of times, until it finally catches something.
About every 10th disc goes right into the trash can, too. I'm verifying all discs on another drive immediately after burning and verifying it in the burner just to be sure, and about every 10th disc fails.

The same goes for (probably) all other storage media such as hard disks. While my oldest hard disk is 19 years (lives in a 12 year old computer) and has no problem whatsoever, this doesn't mean that it won't refuse to spin up tomorrow. Hard disks do that kind of thing in an entirely unexpected manner, and in particular modern drives seem to fail a lot more often than in the good old days.

The only possible exception to this are flash drives, which I honestly don't know about and lack long-time experience (don't we all...). As I understand it, the electrons trapped in the floating gate have no easy way of getting out, unless you expose the flash memory to gamma rays, place it in a MRT, or do any similarly silly thing. Thus, in my understanding, while the number of overwrites is clearly limited on flash, there is not necessarily a limit on its lifetime as such.
So... who knows... a flash drive that is kept in a metal box might live for many decades.