If this short entry will prevent one incorrect apostrophe from entering the world, then it will have done its job. Look at it as some kind of typographical birth control, if you like.
"It's" is ONLY EVER the contracted form of "it is" or "it has".
It's (it is) NEVER possessive (ref: "its job", above and "its box", below).
Whenever you find yourself writing "it's", and it does happen, just take a moment to think: 'Is this "it is" or "it has"?
If it's neither, leave the apostrophe in its box.