Only thing I can say others have mostly commented already. Put the white board inside, If more than one cabin, just leave a note in the fish, and fold it up. Worked well for us. I woke up one morning at 6am, decided to get dressed and take a walk. Soon as I opened the door there was a note from my brother next door saying "mike, up at 5, couldnt sleep, i'm on deck 10", and ended up having a nice little sunrise chat lol. If he had written it on a white board on my door, someone may have erased it just to be a p.i.t.a lol.
On the magnets, I say dont take anything that isnt personalized. somehow, some way, make them all yours whether it be a name, cabin number, whatever. Anything generic is easily taken and relocated on another door.
This happened to my brother. He took an Eagles car magnet, which cost him a good $25 here at home. Someone swiped it from his door (cowboys fan? lol). All the rest of his magnets (which were personal) and all of ours on our door were not touched. You're still going to get the idiots I have read about in other trip reports that think it;s funny to go deck to deck and take 1 ear from everyone's room number ears, or the random snatch. But if you do have them personalized (or at least write your room # on the back in silver sharpie) then if you see one you think was yours on another door at least you can (tactfully) check.
From now on, i am making them as personal as possible. Room numbers, dates, you name it. This brings up a dilemma though. Magnetic paper isnt cheap and who wants to print out magnets for every cruise and never re-use any? Well, at the last minute this trip, the wonderful Jordak made us 2 more magnets and we didnt have any magnet paper left. We did, however have some magnetic business card holders. So, we printed the magnets on plain old printer paper, put them in a clear page protector, and stuck the magnetic business card holders that we cut into 4's on the back side of the page protectors. Worked great, looked great. and now we just take out the paper with the cruise specific dates on it, and we still have the page protector with the magnets ready to take a new page next cruise. Just an idea.