To document our time? Write a diary.

Do the most good? Write a peer-reviewed book about something that isn't World War II and contains footnotes, a bibliography and index.

For the love of god, write legible. Also on the back of photographs. ESPECIALLY on the back of photographs.

Tell us who the people on the photos are.
I just received an important family album this week, and just one generation later, many people cannot be identified anymore.

Leave behind physical evidence of your life.

Future cultural historians are going to have a hard time because so much of our lives, and for some people our entire lives, are on Facebook.

Also the fact that we generate more information than we ever have before, you are going to see an increasing focus on microhistory. ( a long way off obviously.)

Support your local archive and other public history institutions.

Not that others' posts directing you to make sure your own leavings are well-documented and accessible aren't on point, but (in general) archivists are reasonably good at dealing with and collecting historically relevant stuff, no matter what kind of paper you leave it on. And (especially at the small, local level) their biggest problem isn't a lack of stuff to collect, or gripes about handwriting, or even un-labeled photographs. Their biggest problem is funding and public support.

Today's historians rely on collecting institutions to preserve and provide access to the "stuff" (for lack of a better word) that they turn into history. Barring a major paradigm shift, the historians of the future will too. If you really want to help them, working to make sure that those institutions thrive and can do their jobs to the best of their ability is the best way to make that happen (even if the 'stuff' that they wind up preserving isn't your stuff).