Once kids realize they can, it seems they begin to put signs on their bedroom doors. We live in an old house with painted woodwork, and taping things to these painted surfaces usually results in a large square chunk of paint coming off with the tape.
One day I repainted all the doors and installed a simple brass hook, right in the middle, on the outside (hall side) of the kids’ doors about 14 inches from the top. Then I made little two-sided signs that (in fun ways) said to come in on one side and keep out on the other. I had the signs laminated and punched at the copy shop, and strung a ribbon through. The sign hangs neatly on the hook. Now the kids can post their wishes depending on their mood. And no more worry about ruined paint!