Archiving private keys - TLDR version
0. Make multiple encrypted copies. On DVDs (they are better than CDs and Blu-Ray discs; DVD+R are better than DVD-R), paper, cloud services like DropBox, OneDrive, Google Drive, e-mail it to yourself and to your friends, use P2P storage services like MaidSafe, Storj and Sia, etc.
1. Use proper font when printing PGP encrypted keys on paper.
2. Flash memory (SSD, USB flash drives, hardware wallets) is less reliable when not powered regularly (i.e. every week).
3. Use error correction methods like Parchive and ZFS.
4. Print on paper or store on digital media only encrypted data.
5. Your encryption software should use CPU/RAM-intensive KDF (i.e. scrypt with secure options - do not use defaults!).
6. Avoid writing on the hard drive (some printers have too) non-encrypted keys. By default your OS writes all printed pages on your hard drive (and then "deletes" them non-securely). Use Live Linux system like Ubuntu Live or Tails (run from DVD or flash drive in read-only mode; with hard drive disconnected from your computer).