How to use apostrophes with possessive nouns can, at times, be a little tricky. Here are a few tips on how and when to use the apostrophe when forming the possessive of certain nouns:
SINGULAR NOUNS
To form the possessive of a singular noun not ending in an s sound, add an apostrophe plus s to the noun.
Crystal's blog
California's coastline
To form the possessive of a singular noun that ends in an s sound, be guided by the way you pronounce the word.
a. If a new syllable is formed in the pronunciation of the possessive, add an apostrophe plus s.
The boss's approval
St. Louis's airport
b. If the addition of an extra syllable would make a word ending in an s sound awkward or hard to pronounce, add the apostrophe only.
Los Angeles' freeways
For goodness' sake
New Orleans' restaurants
For convenience' sake
PLURAL NOUNS
For a regular plural noun (one that ends in s), add only an apostrophe to form the plural possessive.
Investors' objectives
The witnesses' contradictions
The United States' policy
Attorneys' fees
The agencies' rules
REMEMBER...
Plural nouns that end in s take only an apostrophe, while singular nouns that end in s take an apostrophe plus s, (unless the apostrophe plus s sounds awkward).
The bosses' office - plural
The boss's office - singular
Los Angeles' freeways - singular, but Los Angeles's freeways sounds awkward.
For an irregular plural noun (one that does not end in s), add an apostrophe plus s to form the plural possessive.
Women's coats
Children's toys
A good way to avoid mistakes in forming the possessive of plural nouns, is to form the plural of the noun first, then apply whichever grammar rule above fits:
Singular Noun------------Plural Noun------------------------Plural Possessive
boy----------------------------boys (regular)----------------------boys'
boss--------------------------bosses (regular)------------------bosses'
child--------------------------children (irregular)----------------children's
mother-in-law--------------mothers-in-law (irregular)-------mothers-in-law's
An additional note...
Now to completely confuse you, some constructions can be tricky. For example, the use of back-to-back possessives. Say, for example, you want to refer to the children belonging to two brothers of your father, (i.e., your cousins). The double possessive would be: My father's brothers' children. You could even have a triple possessive with: My father's brothers' children's books. Ouch. Obviously best to stick with your cousins!
Also, a more common problem is the use of a possessive form when ownership is involved. Take the example above of: mothers-in-law's. Obviously we're referring to a possession of two or more mothers-in-law. Rather than, The mothers-in-law's purses, you could opt for: The purses of the mothers-in-law.
ADDENDUM...
I'm including this excerpt, as you will note is also in the reply to Diane's comment. Whether to add just the apostrophe or the apostrophe plus s to possessive nouns, including names, is very controversial; there seems to be no absolute and clear-cut rule:
According to the Chicago Manual of Style, for nouns ending in s: "the possessive of most singular nouns is formed by adding an apostrophe and an s, and the possessive of plural nouns by adding an apostrophe only...this practice is used in conjuction with the way possessive forms are generally pronounced." It goes on to add that, "...feelings on these matters sometimes run high..." thus ending in numerous exceptions and modifications to the general "rule."
Sources: Gregg Reference Manual, CMS, Grammarmudge
