So, storage of private keys is a problem.
Just storing it on your computer isn't such a big problem - hopefully, you're storing it encrypted using software whose encryption you trust.
However, backing them up - that's another issue. Do you trust storing them in the cloud? I don't. Do you keep them on a backup disk or USB dongle? What if they fail?
After looking over the options, I found this post at the ever-excellent StackExchange. Basically, the idea is that you convert the ASCII exported private key to a QR code, print that, note the key ID on the printed copy, and then delete (securely) the exported private key. You then keep the printed QR code somewhere safe - a safe in your house or in a safe deposit box. Then, you can use a QR scanner to convert it back to ASCII (it's still secure because you'll need your passphrase to use the key).
$ brew install qrencode
Export the secret key and covert it to a QR code:
$ gpg --export-secret-key -a | qrencode -l L -8 -o key.png
Then you just print the QR code and then securely delete it.