I think you already propose the best solution: multiply the backup places.
Each support has risks of failing, so the best solution is to multiply, not only in number, but in type too, and in locations as well. You can for example make dvd backups, moreover, and have them in another place, in family, or something.
Because multiplying saves will not have an effect in case of a fire, or even minor incident, if supports are grouped. Another example, justifying the different types, is that each is sensible for different threats. Typically, if your cousin is coming to visit you with his home made electro-magnet, and accidentally makes it fall on a storage place, your hds will most likely be corrupted, but dvds wouldn't. (Ok, I'm pushing a bit the Murphy's law there, but you never know ;-) )
You can also combine with an online storage, but not only, of course, you never know if one day they won't just go away, preventing you from accessing your data again.