The can find complicated routes to places you never thought they could go. You may need to rearrange things slightly to deep them from knocking things over. One way to keep ferrets out of rooms you don't want them in is a baby gate that has plastic floor vinyl stapled to one side. A large piece of wood, or plexiglass cut to fit and anchored in will also work. This means you may occasionally get a little present' in the corner of the room.
Some ferrets even go out of their way to move the litterbox and go behind it.
You must be prepared to spend several hours to several months litter training, and the more attention you can give it, the more likely you will have a litterbox trained ferret. When litter training, the best way is to start the ferret out running around in a small room, such as a bathroom, and work you way out from there. Ferrets will generally go within a few minutes of waking up, or after eating. When you first wake the ferret up, place him in the litterbox. Do not let him out until he has gone.