If I want to use one of the Internet computers when I'm at the public library, I have to enter my 14-digit "library user card number" at the computer. At first I had to look at my library card to get the number, but I have long since learned it by heart. And I didn't need special mnemonic devices; it sort of became part of my "finger memory" from typing it so many times.

On the other hand, I have great difficulty remembering my own seven-digit mobile phone number, because I never call myself!

Incidentally, some historians and anthropologists claim that widespread literacy and the invention of moveable type reduced the average person's ability to remember verbal information. The argument is that back in the days when most people couldn't read and documents had to be expensively copied by hand, huge amounts of info -- the Bible, the Iliad, Beowulf, and so forth -- were learned by oral recitation and committed to neural memory, not to paper. But the ability to memorize book-length epic poems became increasingly rare with the rise of general literacy. (And this historic process is reversed in Bradbury's Farenheit 451 -- when the government bans and destroys most books, individuals devote themselves to memorizing long texts and becoming "human books.")