I've also been having trouble training one of my ferrets (Brickbat; adopted
last fall) to use the litter pan IN the cage.  She has improved remarkably
OUTside of it but still sleeps in the travel carrier every night because
I'm tired of having to scrub out the cage every morning.
 
I am in the process of using the same techniques I used to get her to hit
the pan outside the cage.  The most dramatic improvement occurred when I
started to put her in the pan at random intervals and give her a reward for
just standing in the pan.  I also make her use it when she first wakes up
and always reward her for using it on her own.
 
It may also help to teach the ferret a basic trick, thus reinforcing the
association between human interaction and being rewarded for good behavior
(i.e., training).  Both Brickbat and Seymore made improvements in their
hit rate after I taught them to roll over for treats.
 
If anyone has any other ideas let me know.  Brickbat has been absolutely
the most stubborn ferret I've ever had to litter train.  Fortunately the
two ferrets I've had the longest (Kili and Bumblebee) are literally 100%
with the pan.
 
-mel
[Posted in FML issue 2562]