I found this discussion on the subject:
Unsticking complete sets The most common answers or best solutions? seem to be freezing the cards for sometime (how long they didn't say) I would assume you should also put them in a ziplock bag nobody seems to say if you wrap them in plastic or put them in a bag, I don't think you would just put them in without something covering them. Also the other extreme seems to be a solution "steaming" them separating them above a boiling pot of water. I would use one of those splatter screens on the pot so if you accidentally drop the cards the won't fall into the water, or maybe try steaming from a steam tea-kettle.
I haven't tried either method, haven't thought of them really.