I have one that is completely litter box trained.  When he is loose in our
two story house he will use the litter box every time.  But, it should be
noted that we have 7 litter boxes.  4 for the ferret (including the one in
his cage) and 3 for the cats.  As far as I can tell the cats and ferret do
not use each other's litter boxes although it is possible the ferret is
using the cat's boxes and I just don't know.
 
He didn't always use the litter box consistently.  It required some work on
both our parts.  He originally would go just anywere.  So you start by
confining his activities to a small area that has lots of boxes.  If he
didn't use the litter box we moved the box over to where he went.  When we
ran out of boxes we started cleaning the spots he used with strong
disinfectant carpet shampoo that smelled bad to him.  At that point we had
several boxes in locations that were acceptable, since he had used them.
And all other usable spots smelled funny.  So he started using the boxes.
After a while we started letting him have larger areas and we moved the
boxes farther and farther apart.  But he still continued to use them.  Now
we have him loose in the entire house (minus kitchen) and no accidents.  He
will go from one room to another to use the litter box.  We don't move them
around either.  He can't see well and needs to know where they are.
 
Also note that we never, ever clean his litter boxes with anything but
water.  And we do not make a point of getting them spotless clean.  We only
use cat litter in his cage box.  The other boxes are bare, but smell right
to him.  (The cats were digging in and using his boxes so we started using
less and less litter until we just set them out empty.)
 
It works, but it takes time, compromise and patience!  Don
[Posted in FML issue 1370]