Avoid using abbreviations in running text. Always spell out the abbreviation on first use, unless the term is almost always used in abbreviated form (such as ACT, DNA).
In running text, the names of states, territories, and possessions should always best spelled out (except D.C.) when standing alone and when following the name of a city. In tables, lists, rosters, bibliographies, and mailing addresses they may be abbreviated. When the zip code follows, use the two-letter, no-period postal abbreviation.
"United States" when used a noun should be written out. Use “U.S.” in text only as an adjective.
Example
He is a resident of the United States.
She is a member of the U.S. Senate.
Follow these guidelines for other common abbreviations:
GPA for grade point average in caps without periods
When it is necessary to identify a specific course, use the official course code
Central Daylight Time as CDT without periods
Use a.m. and p.m. in lower case with periods
Abbreviate complimentary titles such as Mr., Mrs., Ms., Dr., but do not use
them in combination with any other titles or abbreviations indicating academic degrees (for example, Dr. Susan Smith or Susan Smith, Ph.D.; notDr. Susan Smith, Ph.D.)
When names of government agencies or universities are used as acronyms, use full caps with no periods ( such as ROTC, MIT)