Few trick I know are used are..
1. Select your fav book and select one word per page note the page number, line number and word number.
2. Use a book cypher to save your private key in a scrambled format.
If you use this method make sure you TEST it to make sure you have made no mistakes and you indeed have the correct info noted down to allow you to decode your PK
Reference :
http://www.drdobbs.com/security/the-book-cipher-algorithm/210603676A book cipher algo uses letters of subsequent words in some text or book as a key to encode a message. Figure 1 is the simplest form, usually called the "running key cipher." In this case, text (usually from a book) is used to provide a very long key stream. T
he book used is agreed upon in advance, while the passage used is chosen randomly for each message and secretly indicated somewhere in a previous message. In this example, we agreed to use J.K. Rowling's Harry Potter and the Order of the Phoenix and to start on page 335, line 28, with the sentence, "Hermione bit her lip and did not answer." We write this text under the plaintext and use it as the running key. The particular message to send is "DRDOBBS." We XOR the corresponding characters of the message and the running key to get the ciphertext 12 23 22 2 11 13 29.
Plaintext D R D O B B S
Plaintext (hex) 44 52 44 4F 42 42 53
Running key H E R M I O N
Running key (hex) 48 45 52 4D 49 4F 4E
Ciphertext (hex) 0C 17 16 02 0B 0D 1D
Ciphertext 12 23 22 2 11 13 29