Use an easily memorable personalized passphrase. One that's easy for you to remember and impossible for anyone else. Repeat it in your mind like a mantra for days and it will stay with you decades later.

Personally I started doing this ~2000. I still remember my first one that I gave up meanwhile because I told it to my GF at the time and we broke up meanwhile.

You can easily top 100 characters this way. Though this is not something you want to do on a touchscreen device, it's a pain there with fat fingers.