'It's' is only used as a contraction for 'it is'. So if you can say 'it is' and keep the proper meaning of the sentence, then it's ok to use 'it's'. Beyond that, gotta leave out the apostrophe.
But yeah, like SoldierNurse said.....it drive me crazy to read something in an email (or an AN post....just sayin') that goes, "We like to give our postpartum mom's Motrin for pain" or something. I want to stab the paper it's written on!!!!!