It is really simple:
-Create an offline wallet with Bitcoin Core and encrypt it (I recommend using PGP on top of the original encryption for privacy reasons)
-Backup this encrypted wallet.dat file anywhere you want. You can put it on several harddisks, usb sticks. You can send it attached to several email addresses.
I keep most of my coins on encrypted wallet.dat, but it's not possible to do day-to-day spending this way. I just use them for coins I don't expect to move for months or years.
Having an offline airgapped computer with Linux: This looks like the safest option to me. The problem: You need to pre-sign the transactions and Bitcoin Core does not have good support to do this so you are stuck with making raw transactions by hand which is a bit of a mess, then you need to pass this raw transaction into your online node. Armory has a nice GUI to do this but I wouldn't trust it too much. Electrum.. wouldn't trust my main stack to be held by an Electrum wallet (too paranoid about the seed being derived somehow)
I wasn't aware that Armory had a GUI for offline signing. Any opinions here on Armory vs. Electrum? I use the latter, but I'd feel a lot more secure if I could do this with Core. Also, is it true that Electrum's seed might be easier to derive? What's the difference among the different HD wallets regarding seed security?