Shari,
All of my little critters absolutely *love* to get between the mattress and
the fitted sheet.  While it is completely annoying, in our case it also
seemed to endanger the little beasties.  They began digging at our mattress
pad (filled with cotton batting) and when they got through that they found
our foam "egg-carton" pad underneath.  Obviously we had to find a way to
stop this.
 
We have a queen size bed, so I bought a king size flat sheet and now I use
that as a bottom sheet.  While it isn't perfect, the fuzzies have to work a
lot harder to get it untucked to get under it, so they seem to be slightly
deterred.  I have also "duck" taped it to the mattress (underneath) to make
it a little more difficult.  I don't know if a waterbed sheet is bigger, but
if it is that would be better as the king size sheet just *barely* does the
job.
 
Also, I pushed a small chest I have up against the end of the bed to make it
easier for the ferrets to get in bed with us.  I found that if they don't
have to *fight* and climb to get up there, they are much calmer and more
pleasant when they do come up, since it is completely on their terms.  I
hope this helps, and if anyone else has any other suggestions, *I'd* love to
hear them too!
 
     Later,
     Shelly and the 3 weasels
[Posted in FML issue 1609]