To help remember credit card PIN numbers, I used to make a dummy entry in my filofax, like, e.g., Norman West or Barclay Plastics. I would enter a fake phone number, say 0207 314 7654, the last four digits being the PIN number. To make the entry look more authentic, I would also enter another random number for FAX.
When I upgraded my Barclaycard, I was allocated a new PIN number. I went to the entry in my filofax to amend the phone number, only to find that my new PIN was precisely the fax number I had invented some years before !!