Getting apostrophes right is not hard when you remember they are used to indicate possession or there are letters missing in a word. This week I saw:
… you’re brother …
Your is one of those words that indicates possession and doesn’t take an apostrophe. The apostrophe indicates there are missing letters so that you’re means ‘you are’ which is clearly wrong in that context.
See my presentation on Slideshare with more info on using apostrophes.