I agree about documenting their lives.. but as Corky pointed out.. must of us will not do it, we seem to put it off.
I really think that the best way to do this is to pick your time, and then sit down with a nice cuppa or it's equivalent and start talking whit a tape recorder set up perhaps a little to one side and slightly behind them?
I would have someone test the sound pickup beforehand.
Then you can play the tape at your leisure and type it all down. This could become a wonderful sharing, as I doubt if it can all be copied in one evening.. so this could become a Sunday ritual.
But don't just think about your parents.. look to yourself.
We all think that things will never change, but they do. And especially in this day and age.
Stop and think about the changes that you have already experienced. then think about how much more it will change by the time you are in your late 60's or 70's etc.