The Nitpicky English Major's Guide to Grammar: Your or You're?

"Your" and "you're," like "it's" and "its," are easy to mix up.  They sound the same (unless you're a weird hybrid of northern/midwestern and southern accents like me and pronounce "your" like "yer") in conversation, so transferring them to the written word correctly can get them mixed up.  However, they are completely different in meaning.  "Your" is a possessive adjective, meaning that X belongs to you.  "You're" is a contraction of the pronoun "you" and the verb "are," and means "you are."

A quick substitution test (like with "it's") can be performed to determine which form to use.  

If you could replace "your/you're" with "you are" in a sentence, then "you're" is appropriate.  Here's an example, with the substituted words in bold.

After reading this post, you're going to know the difference between "your" and "you're."

After reading this post, you are going to know the difference between "your" and "you're."

If "your/you're" cannot be replaced with "you are," then "your" is the correct form.  You can see the difference in this example, again with the substituted words in bold.

After reading this post, you will check your spelling.

After reading this post, you will check you are spelling.

Does it make sense?  No, you can see that it doesn't.  It would sound strange to say it out loud.  In this context, the spelling belongs to you--it is yours.  

Now, wasn't that easy?

By the way, "ur" is never an acceptable form of "your" or "you're," except maybe sometimes in texting when brevity is usually of greater importance (though I always spell words out).  I actually have seen it in an academic paper that was submitted for a grade.  Needless to say, the student lost points.  The way you write on twitter, facebook, or in text messages is definitely not how you want to write for something that gets graded or that will be circulated in the workplace.  

Well, this is going up a little later in the day than I meant, but I've been busy working and getting some photographs of my little town before we leave in twenty-six days, not that I'm counting.  But hey, you'd count down, too, if you knew that you would be moving from a town with no bookstores to a city with at least ten, wouldn't you?  Today my boss asked me how I am about grammar and proofreading before I looked over some documents.  I laughed and said that I'm nitpicky.