OK, sometimes it's typos... but some people genuninely seem to struggle with this, although it's quite simple. There's no need to know about "posession" and "contractions", like they'd have you believe at school.
REPLACE THE "YOUR/YOU'RE" WITH "YOU ARE" AND SAY IY OUT LOUD. If it makes sense, use "you're" and if it doesn't make sense use "your". Simple, eh?
YOU ARE going out tonight.
That works. You're going out tonight.
YOU ARE going to spend the night in Misc.
That works. You're going to spend the night in Misc.
YOU ARE black gerbil1.
That works (even if you aren't him). You're black girbil1.
YOU ARE handbag.
That doesn't work (unless you're a user called "handbag"). **Your** handbag.
Now go in peace and be, erm, peaceful.
your has been drinking tw0 much jack daniels awd lad