indeed this is a favorite topic of the Long Now Foundation – how to prevent a “Digital Dark Age” without recording every possible bit of data.

I myself have chewed on how to habitually purge data, so we can continually create space (http://showerinthedark.blogspot.com/2007/10/digital-recycling-game.html). we could have ritualistic inbox cleansing parties and somehow tag our files with expiration dates, but is there a way to automate this process? is there something about information that remains relevant for 1 day vs. 1 month vs. 1 year vs. an indefinite period of time that AI could sniff out and use to determine, in your words, what to remember? if there were, it would happily save space, money, and of course, energy.