In just a few more months, the 1940 census will released. It won't be indexed, although I'm sure several genealogy websites will begin immediately. Unless you're satisfied waiting until indexing is complete, it's time to start preparing so you can extend your lines.
Make a list of individuals you wish to find in the 1940 census. This should include people that you found in the 1930 census that should still be living as well as anyone born between 1930 and 1940. Don't forget to include female children from the 1930 census who may now be married. If you haven't check to see if they're married yet, now is the time to do it..
Determine a probable location. In families that tended to stay in the same place, this will be easier than with families or individuals that frequently moved. For frequent movers, check city directories, WWII Draft Records, and records of vital records or immigration that occurred close to 1940. This can help narrow down possible locations to search.
Find the enumeration district. If the person appeared in the 1930 census and you're relatively sure they didn't move by 1940, you can use Steve Morse's Census ED Converter. If you have an exact address from another source (aren't you lucky?), you can use the ED Finder or ED Definition tool at the top of that page.
Make your list. Put it in whatever format you prefer-a handwritten list, a Word document or a spreadsheet. I would suggest grouping individuals by location to make your search a little easier when the census is released. It's more efficient to locate all wanted individuals in a single area rather than jumping back and forth.
A final note. If you haven't yet volunteered in the genealogy community, this is your chance to give back in a big way. FamilySearch is seeking volunteers to index the census.