You can never be quite sure when you put up one of those ‘Do Not Disturb’ signs that you will not, in fact, be interrupted. To get rid of that nagging doubt that lingers in the back of your mind there is really only one perfect solution: prevent someone from entering, physically. One clever way to be sure you are not bothered in your room is to have a retractable door handle.
For one thing, these door knobs are very clearly signs that someone is in fact occupying the room – since they have to pull them from the inside in the first place. This saves confusion and trouble on both sides of the door. Also, engaging their privacy mode can be done without even opening the door and stepping outside to hang something that might or might not remain on the front of the door to let people know you are busy.