For most singular nouns the ending �s is added; e.g., the cat�s whiskers.

• If a singular noun ends with an /s/ or a /z/ sound (spelled with -s, -se, -z, -ce, for example), practice varies as to whether to add �s or the apostrophe alone. A widely accepted practice is to follow whichever spoken form is judged better: the boss�s shoes, Mrs Jones� hat (or Mrs Jones�s hat, if that spoken form is preferred). In many cases, both spoken and written forms differ between writers.

There is a lot of controversy on this all over the web.  I tend to follow Strunk and White (probably the most used grammar guide in US Universities).

 I found this interesting little tidbit at:

http://www.legalwritingpro.com/articles/C15-feeling-possessive.php

When the Supreme Court reviewed Kansas v. Marsh last term, the justices didn�t just split over whether to uphold a Kansas death-penalty statute.

They also disagreed over a usage issue that has driven many lawyers to blows: Whether to write �Kansas� statute,� as Justice Thomas did in his majority opinion, or �Kansas�s statute,� as Justice Souter did in his dissent. �

Souter may have lost the substantive battle, but he won this stylistic war: Nearly all authorities agree that if you want to make a possessive out of a singular noun like Kansas that ends in an s, you need to add �s at the end. Just call it �Ross�s Rule.�

Better yet, remember it as Strunk & White�s �First Rule,� which it�s been since that classic�s first edition:

Form the possessive singular of nouns by adding �s.

Exceptions: Classical or biblical names, such as Moses, which take only an apostrophe: Moses�.