I prefer to put a poster on my door that says CRUISE-ADDICT.COM. It is so much easier to find my cabin. If I forget my poster..... I use a ship post card that is given to you in the cabin. I carry my tape stuff with me.
I have seen the ship workers wash the balcony windows once per week during the week. So if you have a banner it will get all wet. You are not supposed to leave anything out on the balcony because it will get wet when they wash the window and the balcony. I saw it in the NCL Cruise ship.