In general, eliminate periods unless the abbreviation spells a normal word:
AMA DOE OPEC YWCA in. = inches (not in) no. = number (not no)
Units of measure that are abbreviated do not have periods, and the same abbreviation is used for both the singular and plural:
cm ft lb m mm oz yd yr
Some abbreviations still require periods:
a.d. a.m. b.c. Dr. e.g. etc. i.e. Mr. Mrs. Ms. p.m. pp.
When using periods in abbreviations, do not put a space between the period and the next letter:
a.d. (not a. d.) i.e. (not i. e.)
When an abbreviation ends a sentence, use only a single period:
The meeting will begin at 10:00 a.m.
If the sentence or clause ends with punctuation other than a period, the other mark of punctuation follows the period of the abbreviation:
Will the session be over by 4:30 p.m.? After 7 p.m., the hotel restaurant is often filled to capacity.