Export your private keys, AES encrypt them with a secure passphrase (use base64 or ascii armor encoding so that you can write it down), save in text file on your computer + in usb/phone/safe cloud storage/email attachment and print out the encrypted string and keep in wallet/drawer/safe.

You can AES encrypt text files with GPG: https://steemit.com/privacy/@retprogramisto/how-to-use-symmetric-password-encryption-with-gpg

Dont save your private keys anywhere unless they are encrypted.

If you have a large amount its better to store it in multiple addresses so that if one private key is compromised the others could be safe.

You could also use hardware wallet such as Trezor or Ledger, but still backup your encrypted private keys somewhere else in case the hardware breaks or you lose it or it is stolen.

Brain wallet is insecure, dont use it.