To write Santa for goodies or with wish lists you should address your letter to Santa Claus as follows:And here I thought you needed the Ark of the Covenant to send messages to God. Link (Thanks, Gribblet!)
The USPS will see that the letter is received at the proper place. Please ensure to include the return address on the letter itself! Letters to God can be addressed in the same way replacing "Santa Claus" with "God".
Update: Lukethelibrarian sez, "the reason why the USPS wants a real return address, city and state on there (or tries to associate one) is so that the letter can become a part of Operation Santa Claus, in which many postmasters across the country participate. The program varies from one post office to another, but often includes responses to the kids (written by volunteers) and sometimes real gifts, through partnerships with United Way and Toys for Tots. "