Paint the door, doorknob and trim the same color as the walls. Although the door is still in plain sight, the solid color scheme makes it less obvious.
Cover the door with wallpaper to match the decor in the rest of the room. If the top half of the wall is wallpapered and there is a chair rail and wainscoting below, wallpaper the top half of the door and then add a wainscoting panel and chair rail to the door.
Paint a realistic mural on the door that blends in with the surroundings. For example, you can paint a small window on the top half of the door and paint the bottom half to look like the walls and floors, including features such as a small table with plants.
You can even purchase painted tarps to disguise garage doors with images such as horses, beach scenes and subway stations.
Remove the door knob from the door to make it appear more 2-dimensional. This is best suited for doors that you won't need to open, but you could also install a door that swings in to open.
Hang curtains over the door, using the same curtains and hardware used throughout the rest of the room. Choose floor-length curtains and keep the shades drawn, and no one will assume there is a door behind the curtains.
Cover the door with a large piece of furniture, such as a cabinet or bookcase if access to the door is not required. Removing the doorknob and door trim may be necessary to push it against the wall, limiting the view behind the furniture.