If you have a lot of money in your cryptocurrency wallet you should at least consider using a hardware GnuPG "smartcard" like nitrokey or yubikey. This way even if they get your passphrase they still need access to your hardware token (and vice-versa). There are also hardware wallets specifically meant for cryptocurrencies which I suppose have the advantage of not requiring you to decrypt your wallet on your drive before you make a transaction.
I'd embed them into QR codes, print them out on high quality acid-free paper and put them in a fire safe.
I don't trust anything electronic to store data in the long term, I do trust that a QR code will be readable in 50 years even if you have to decode it by hand (or at least write a program to do it) (in a temperature controlled safe with no UV I'd expect laser toner to last 50 years).
Other methods I can think of would be etching the QR code into a stable material (glass for example).
That said I stay miles away from cryptocurrencies, I find the technology interesting but the actual 'market' hilarious.
