Standing alone state names are spelled out: Michigan is a wonderful place to live.
It is not necessary to include the state with city names that would not easily be confused. Locally, this includes Marquette, Houghton, Detroit and Green Bay. For larger cities, this includes such cities as San Francisco, New York, Seattle and Las Vegas.
When used with a city, state names are abbreviated using the abbreviations in the Associated Press Stylebook, and the state is set off by commas: Susan moved to Baldwin, Md., and David came from Sheridan, Ore.
Use the postal abbreviation for addresses, except when addresses are included within body copy. In this case, the preferred form again is the AP style: Mich., Wis., Ill., Ind., Minn., etc.
Note: Washington, D.C. should be written out to distinguish it from the state. States with five or fewer letters are written out as in Ohio, Maine, Texas.
see also punctuation - comma; states