I take it that the door was frozen to the seal. In that case -> clean the seal and apply "gummi-pflege". That stuff repels water. No water - no frozen door.
Frozen windows (somebody mentioned that too):
When the windows wont go down (on coupes and convertibles) they are usually frozen to the window seal right behind the trim. What I usally do is spray WINTER-windshield-washer-fluid on the seal (cheap -> 5 Liter = 3 Euros). I always keep a spray bottle filled with that stuff in the trunk because I also use that stuff to de-ice the windshield and the windows. I hate to scratch the ice off because it can leave scratch marks on the glass (you wont see scratches during wintertime cause its dark but you will notice the scratches in the summer). In case of emergency (like when you dont have washer fluid handy) you can also use a credit card to sperate the seal from the window - you can imagine how. Ive done that a couple of times, too.
Last year I "treated" the windows (around the area that touches the seal) with the same stuff I put on my windshield (to repell water). It helped quite a bit.
Last edited by silyb0y; 11-30-2012 at 10:45 PM.