I used to have trouble remembering rarely used, yet important, security codes. With a bit of effort I conjured up an image in my mind of four words representing the four numbers in the PIN, and as the human mind is much better at remembering images than digits, I have not forgotten that security code since.
To make this process easier I created an online mnemonic generator, which in its initial form works purely for 4 digit codes. In theory it should produce some sensible and memorable combinations. Usually the key is that the number of letters in the word corresponds to the particular digit, but I had to take liberties with the '0' and '1' digits. For example, of and nought may mean '0'. Fortunately, it's easy to remember these particulars.
In the future I may extend this to longer codes.