When we were on Valor in April, we brought along fridge magnets for our doors. They "stuck" ok, but they kept disappearing! Maybe because they were a moose and a beaver holding Canadian flags, I don't know, but I replaced them a couple times before giving up!
I recommend sticky tack - it is like putty but comes off surfaces fairly easily. I read the room steward is responsible for the door-please be courteous and clean any residue hence I'd avoid really sticky tapes.
I did see a neat door decor last trip--it was a metallic green grass and palm trees like the old "bead curtains" really caught my eye as we walked down hall. Definitely knew where your room was with that one.
I also saw a dry erase board--strange messages get posted on it!
Adventure of the Seas RC April 2008
Carnival Liberty April 2007
Carnival Miracle April 2005
Carnival Destiny June 2001, July 2005
NCL Southward June 1985 Honeymoon!
Last cruise was on the Voyager of the Seas and the door signs we made were done on magnet paper and they stuck great. Of course they got moved around from door to door but that was great! I don't have any problems with that.