sometimes it helps using the door on the other side. Had that problem last winter: the passenger side door was less affected than the drivers one. and way too often it's not the lock that freezes but the mechanism inside the door and the part that makes contact with the cars frame. so be sure to apply lots of grease to the whole mechanism. And apply some vaseline or smiliar to the door gaskets, as the tend to freeze too. the more the merrier. and the better they seal, the less moisture you'll get into your door. Depending on your car it may makes sense to add an additiional outside seal to the door, so that less water can enter the door lock mechanism.
if you have the problem every day I guess the only thing that really helps is drying the shit out of that door. And then you can thinkg about on how to prevent new moisture entering it. but first of all you need to get rid of what's already inside.