When we were kid we used a methods to remember several character name from our history lessons. We use those name and make a rhyme that we memories. This methods works because it's easy to remember a rhyme than some random words. If you do not wish to write your passphrase in anywhere in fear of compromising it try to make a rhyme or poem with it only for you. Mnemonic phrase usually some random word so give them a meaning so that you won't forget and do not tell it to anyone. Even if you write it down in some place no one will understand what is it if you do not tell them.

Though this is a retentive questions but sometimes it brings new ideas so thanks anyway.