Number one, we've just begun
Rod Dreher revealed recently that he couldn't come up with more than six of the Ten Commandments from memory. He also pointed out the irony of this fact coming from someone who often gets on his "high horse about theological ignorance," so I won't pile on.
I've mentioned before that, while I haven't retained everything I learned at my evangelical grade school, I do recall a catchy song for remembering the U.S. presidents in order. (This came in handy this past Presidents Day in an internal Century office competition.) We also performed a lot of musicals, including the popular '80s Christmas program Angels Aware—which features the (classic!) Ten-Commandments-remembering song "The Perfect Ten."
Less helpfully, the lyrics also offer a gloss on each commandment:
Number three: God's name should be never spoken in jest.
Number four: the sabbath's for our worship and for rest.
Number five: we all should strive to honor father and mother.
Number six: don't get your kicks from killing one another.
My sisters and I were recalling this on Facebook a while back. I pointed out that, while the biblical commandment against taking the Lord's name in vain is not the most straightforward of the bunch, I'm pretty sure "God's name should never be spoken in jest" is doing it wrong. (No jokes about God!) One sister added this: "I've always thought it was okay to kill someone as long as you didn't get any kicks from it." The rhyming, not the meaning, seems to be the priority in Angels Aware.
Anyway, years later when I saw The Decalogue, some of my college friends relied on me for quick reference as to which commandment Krzysztof Kieślowski was dealing with in a given episode. At which point I had to confront a new (to me) issue: the lack of consensus on how exactly the commandments passages should be split off into a list of ten.
Numbering and interpretation aside, it's very useful to have a song's help in committing something so important to memory. I wish that trick were as easy for grownups.