Like if someone were to write down the kind of things that would likely slip through the cracks and be lost to time, what would they write down?
Its hard to know but if you kind of person who is on reddit arranging for you personal private data to be preserved and released after death could be useful.
Aside from that helping organizations like the Internet Archive and the LoC in their archiving may also be help.
Diaries and letters have previously been quite useful but frankly I suspect future historians will not have a dearth of records barring some calamity so unless your are building a fortified tomb with a large data vault to survive some cataclysm just releasing your personal data should be sufficient.
If your really adventurous you could try to backup documents like are likely to be destroy or permanently interred in some manner by robbing the CIA, or FSB or more realistically voting for people who work to strengthen organizations like the national archives and Library of Congress.