Since "grammar" and "correct uses of apostrophes" are two of the most searched for terms on this site, I thought I would draw your attention to this rather fabulous resource courtesy of The University of Melbourne.
Consistent apostrophe misuse is one of the most common errors I see in spec scripts and it's a real shame, since it's easy to put right if you know how. The reason this resource is so cool is because it's concise and to the point - plus it's a pdf so you can download it and go back to it later. It also has some handy links too, including one to the brilliant BBC Skillswise website, where you can get all manner of games and worksheets to help you improve with things like grammar. Click here to go direct to Skillswise.
So save it to your desktop now and banish this troublesome and pesky error from your scripts forever: click here for the apostrophes pdf. Enjoy!
UPDATE: Here's another good link about apostrophes from the marvellous Rich... Thanks Rich!