>You should have secure copies of your master key. Especially if you've invested time in building up your web of trust. Backing up encryption keys is also useful. If they're lost, data cannot be decrypted later. Signing keys are the only ones which are completely expendable.
The paperkey tool will produce a human-readable text file that you can print out. I found that to be inconvenient and settled on QR codes instead. They're ubiquitous, machine-readable and have built-in error correction.
Where can I read to how to do that?