Bob the Angry Flower tells it like it is (you idiots!):

That's it Folks! That's all y'gotta remember! Can you substitute "it is"? Then use "it's". If you can't DON'T!!!

Example: The cat hurt it's feet > The cat hurt it is feet. > WRONG!

This one's stupidly simple, people!!!

Biggify the picture here: Link, check out more of this fantastic webcomic: Bob the Angry Flower by Stephen Notley (apparently, Bob has an intense hatred of the incorrect us of apostrophe to form plurals - a grammatical crime I've committed from time to time) - Thanks Lee!