When should I use apostrophe-S?

When the singular or collective form of a noun ends in -s, there are two ways to form the possessive. The preferred method is to add apostrophe-s (-'s). However, some publications add only the apostrophe.
Paris's lights reflect beautifully off the Seine.
The Jones's car is parked in the driveway.
The class's attendance this year was excellent.

In most cases, apostrophes should not be used to form plurals. Some publications use apostrophes with dates, but this should be avoided if clarity can be preserved.

We began the practice in the 1980s.
He followed the four Cs when buying her diamond: color, clarity, cut, and cost.
I have all of Aerosmith's CDs.
There were three Cindys in my third-grade class.

However, some coined phrases do use apostrophes to indicate plurals (note that the apostrophe is necessary here to avoid confusion). Also, common abbreviations that have more than one period require the use of an apostrophe.

I told my daughter to mind her p's and q's when she went to her friend's house.
Elizabeth has earned three M.A.'s and two Ph.D.'s.

Use apostrophes with forms fashioned with verbs that are also abbreviations.

O.K.'d
O.K.'ing