Retired Fed employee here. In one of my jobs, a limited number of us needed access to a particular area, and security needed to know who went there and when. So they told us our PIN would be the 1st, 3rd, 6th, and 9th digit of our SSN. As you might imagine, most of us would have to count our way thru the PIN.
I use one of two PINs derived from the telephone number I grew up with (just the one, in those days), which was last used by my family in 1969. Nobody remembers that number but me, not even my sister. So I don’t see any reason to scramble or jump through any hoops to disguise what I’m doing. If anyone were ever to get hold of one of my PINs (which if it happened would not depend at all on how the number was derived) I would find out eventually, and change it (to the other one).
I operate on the assumption that no-one is using any sophisticated spying techniques on me, such as one sees in movies, because if they did, any of the defensive moves I might have thought up would be useless anyway. After all, there are only 10,000 possible combinations for four digits, if anyone really wants to break one of my PINs they can probably do so, provided the put forth the effort.
I’ve seen several comments about picking a word that converts to a series of numbers on the keypad.
Back in the “good old days” when ATMs were new, I filled out my application for an ATM card and used my brother’s name for the requested PIN. Unfortunately, the name had an “I” in it and the bank read it as a “1” instead of the letter. As soon as I got my card, I rushed to use it and found out that my PIN (brother’s name) did not work. Back then, it was going to be a royal PITA to change the PIN and, of course, the bank wouldn’t tell me what the actual PIN was. For some reason, I had the inspiration to try a “1” inplace of the “I” and it worked. No new card required…everything A-OK.
I used that PIN for everything I could for over 35 more years.
My PIN for everything is 1234. Everyone knows its the worst PIN to choose, so no one will guess it. 
Actually, I use physical constants and choose a section of one. So all I have to remember for a given PIN is which constant and where to start in the number.
Since I share my personal identification PIN number with Mrs Cad, I made sure its not one I use anywhere else.
I still use the number that was randomly assigned on my first ATM card. It happened to be an anagram of the number on my daughter’s mailbox at college, so was easier to remember.
Which clearly, should just be AT. Because ATM is also redundant. You’d expect, what, an automated teller person? Of course an automated teller is a machine.
One ten-sided die four times.
Saying PIN number is redundant and repetitive.
Less redundant than being the umpteenth person in the thread to state it, amIright?
Keypad positioning. A person might, for example, choose 7931 because it’s the four corner numbers on their keypad in clockwise order.
It’s also like how language works. The redundancy is to make it immediately obvious what you’re talking about and, say, not a sharp instrument, like how some people say “ink pen.” Redundancy is not to be eschewed in language automatically.
Being a teacher of AP European History for over 20 years, I usually have PINs from significant dates. Really not that limiting, either. Especially if you go for the obscure-but-not-to-you dates, like choosing the 1419 defenestration of Prague as opposed to the 1618 event.
Eschew obfuscation!
I think “redundant and repetitive” was a redundancy joke.
Czarcasm apparently felt it was a redundant redundancy joke.
Mine is a particular word shortened to 4 characters, converted to numbers via the standard phone layout.
What makes mine hard is that the original word is the name of the primary character in a book I never wrote…
So you keep it written on a postit on the bottom of your keyboard, right?
I use mathematical constants for my PINs. Get a hold of π, e, γ, φ, sqrt(2) … and select a sequence of 4 digits from inside it.
I read that and immediately remembered the number of my college mailboxes. Hadn’t thought of those numbers in years. Weird how memory works.
I had two mailbox numbers, as the college post office moved during the summer between my junior and senior years. I think I used the first mailbox number as a PIN some years ago, but I’m not currently using it.