U.S., USA
Ph.D., BA, AB, MA, M. Litt., MD
NATO, AIDS, CEO
e.g., i.e.
etc.
A.M., P.M.; or a.m., p.m. (A.M. and P.M. are made with SMALL CAPS for a more formal and easier-to-read look)
Spell out the full name the first time it is used in text, and place the abbreviation or acronym in parentheses immediately following. In lengthy pieces, repeat the name occasionally if the acronym is unfamiliar and the name hasn’t been used in several paragraphs or pages.
Capitals versus lowercase? Depends on usage. Longer acronyms tend to become lowercased with repeated use, i.e., WASP vs. Wasp
Abbreviations, Plural
Abbreviations without periods, and numbers take s, no apostrophe. Apostrophes may be used if misreading is a possibility.
BA, BAs; in twos and threes; ’80s and ’90s
Abbreviations with one period usually add the s before the period:
ed., eds.; yr., yrs.; Dr., Drs.
Abbreviations with more than one period use apostrophe s:
Ph.D.’s