The digital images of the 1940 census are set to be released on April 2, 2012. Have you decided who you'll be looking for? Do you already know where they were living? The census is indexed to the enumeration district level, so if you have a location, you can find the enumeration district using tools available on the 1940 Census page at the National Archives website.
There is also a 3:13 minute video at the 1940 Census website which gives a great introduction to the project, as well as some tips. The web address is: http://www.1940census.archives.gov/. This is also a National Archives page, but has a special url to access census info directly.
And don't forget to sign up as a volunteer indexer. The more, the merrier! https://the1940census.com/