WikiHow has a great lesson in proper apostrophe usage—the Achilles' heel of bloggers everywhere (except here, of course). For example:
Avoid using an apostrophe to indicate a plural. The incorrect use of an apostrophe to form the plural is called the greengrocer's apostrophe, since grocers are often the worst (or at least the most visible) offenders. If you have more than one apple, then write apples, not apple's. If you cannot replace the word with "his" or "their" and if it isn't a contraction, then an apostrophe should not be used.
I must be a major grammar geek, because I love this stuff. If you strive to keep your writing as clean and accurate as it can be, this is definitely worth a read. Meanwhile, check out this list of 10 common grammar mistakes. Photo by malingering.
How to use apostrophes [WikiHow]