• If you can’t find a household in records for a database site such as Ancestry.com, you can browse by ED (in Ancestry.com, choose a census year, then scroll below the search box to pick a state, county or township; a ward; then an ED).
  

• Enumerators didn't always proceed through their EDs in orderly fashion: Rather than go down one side of the street and up the other, they might cross back and forth or double back to places where no one was home. But you can compare a census return to a map of the corresponding ED to plot the neighborhood and see who lived next to your relatives.

• When the 1940 census comes out on microfilm in 2012, a name index won’t be available right away—but while you wait, you'll be able to find the records using the ED. Update: Good news! Name indexes may be available immediately after all. Click comments (below) for details.
  

• Stephen P. Morse’s Web site has an ED finder for the 1910 to 1940 censuses (mostly for urban areas). Scroll down to the census section of his home page to find it. 
  

Morse’s site also offers a tool for translating among EDs from 1910 through 1940.

• NARA has put ED descriptions for each census on microfilm. Series A3378 has EDs for the 1900 through 1940 censuses; series T1224 goes back to 1830. Update: Click comments for details on ED microfilm, too.
  

• The Family History Library also has ED microfilm, which you can rent for viewing through your local branch Family History Center.