Passwords. I always remember my army pay number, two of my Dad's motorbikes from my childhood (can't remember the car and van that I've owned for the last nine years) and my Mum's and Gran's Co-Op numbers (if you don't know, you are not British.) Any of these can be expanded with initials or birthdates, etc.

I do take JohniK's point about IT people, though. I worked in a firm where we were required to change our password weekly. I changed mine very weakly, trying to drive him mad.