In order to get you ready for the opening of the 1940 census (April 2, 2012!), we're going to break the steps down one at a time, so that, instead of waiting, you can methodically get all of your ducks, ah, ancestors in a row. Remember that, although the census will be available at the National Archives and Records Administration site immediately, and several other sites in quick order, the census will not be indexed. Being prepared will save you a great deal of time once you finally get a chance to browse.
Your first step is to identify for whom you will be searching. Parents or grandparents? Siblings and cousins? At this point, create a file, paper or digital, on each person or family group, noting names, ages or birth dates and places, and where you think they might be living in 1939 or 1940. Check the files you already have, old letters and address books to see if you have possible addresses for them.
That's it! That's step #1. Step #2 will be coming soon!