Excerpts from a useful grammar website: Handy for translators and expats alike.
Mrs., Mr., Ms., Prof., Dr., Gen., Rep., Sen., St. (for Saint)
Miss is not an abbreviation.
Ms. is not an abbreviation, either, but we do use a period after it — probably to keep it consistent with Mr. and Mrs.
The plural of Mr. is Messrs. (We invited Messrs. Carter, Lincoln, and Ford.) The plural of Dr. is Drs. (We consulted Drs. Carter, Lincoln, and Ford.) The plural of Mrs. is Mmes or Mmes. (with or without the period).
In most formal prose, we do not use titles, abbreviated or otherwise, with individuals. Ms. Aaaaa Bbbbbb is simply Aaaaa Bbbbbb, and after the first use of her full name, Bbbbbbbbb will do .
Titles after names:
Sr. Jr. Ph.D. M.D. B.A. M.A. D.D.S.
The Chicago Manual of Style recommends not using a comma to separate the Jr./Sr./III from the last name, but you should follow the preferences of the indivdual if you know those preferences. If you list a "junior" with his spouse, the "Jr." can go after both names, as in "Mr. and Mrs. Arthur C. Banks Jr." or "Mr. Arthur C. Banks Jr. and Gloria Banks — but not Arthur C. and Gloria Banks Jr. You should avoid using a "Jr." or "Sr." when you have only the last name — Mr. Banks Jr.
Notice that U.S.A. can also be written USA, but U.S. is better with the periods. Also, we can use U.S. as a modifier (the U.S. policy on immigration) but not as a noun (He left the not U.S. but U.S.A.).
Terms of mathematical units: 15 in., 15 ft, 15 kg, 15 m, 15 lb.
When modifier: a 15-ft board, a 6-lb line, etc.
790 BC but! AD 78 n in SMALL CAPS.
Latin terms: etc. (et cetera — and so forth), i.e. (id est — that is), e.g. (exempli gratia — for example), et al. (et alii — and others).
There is a difference between acronyms and abbreviations. An acronym is usually formed by taking the first initials of a phrase or compounded-word and using those initials to form a word that stands for something. Thus NATO, which we pronounce NATOH, is an acronym for North Atlantic Treaty Organization, and LASER (which we pronounce "lazer"), is an acronym for Light Amplification by Stimulated Emission of Radiation. FBI, then, is not really an acronym for the Federal Bureau of Investigation; it is an abbreviation. AIDS is an acronym; HIV is an abbreviation. URL is an abbreviation for Uniform Resource Locator (World Wide Web address), but many people pronounce it as "Earl," making it a true acronym, and others insist on pronouncing it as three separate letters, "U * R * L," thus making it an abbreviation. The jury is still out (I vote for Uncle Earl).
Using articles with abbreviations and acronyms:
One of the most often asked questions about grammar has to do with the choice of articles — a, an, the — to precede an abbreviation or acronym. Do we say an FBI agent or a FBI agent? Although "F" is obviously a consonant and we would precede any word that begins with "F" with "a," we precede FBI with "an" because the first sound we make when we say FBI is not an "f-sound," it is an "eff-sound." Thus we say we're going to a PTO meeting where an NCO will address us. We say we saw a UFO because, although the abbreviation begins with a 'U," we pronounce the "U" as if it were spelled "yoo." Whether we say an URL or a URL depends on whether we pronounce it as "earl" or as "u*r*l."