GPG private key – I like to store mine on paper (security.stackexchange.com)

An alternative is to use Paperkey [1], which creates a text based encoding of your private key that can be scanned in and OCR'd. I believe there is also some redunceny similar to using a QR code. This method is designed for long term storage.

[1]: https://www.jabberwocky.com/software/paperkey/