As I've mentioned before, grammar is full of gray areas; unlike math or science, in which A+B will always equal C, English language usage contains both exceptions to the rules and situations for which the explanation almost makes things more complex. Such is the case when it comes to denoting possession when words end in "s."

I frequently get questions about this issue. I have always used an apostrophe alone (s' rather than s's), but I learned some rather surprising (and confusing) things after doing some research on the subject.

• For singular nouns, there's no "correct" way. "The Elements of Style" recommends using the 's, but other "experts" note that it's better to leave out the extra s if using it results in clunky pronunciations. (Las Vegas's famous strip versus Las Vegas' famous strip--both are OK, but the latter is easier to say)
• For plural nouns, an apostrophe alone works. To create possessives of words that have been pluralized by adding an s, it's appropriate to merely add an apostrophe (the patients' meals).
  
• When in doubt, try to rewrite. If you're confused about whether to use an apostrophe alone or add 's, try to reword the sentence so that's not necessary. (our business' tax returns or our business's tax returns can be rewritten: the tax returns from our business)
  
• Be consistent. If you choose not to add the 's in instances when you must denote possession for a noun that ends in s, do so throughout your document. There's nothing more confusing to readers than inconsistency.