Honestly, I've found that the best method is to keep bedding and litter
clean and that's it.

Some people bathe their ferrets often, but that strips the oil out of
their fur, and causes them to produce more to make up for it. In the
end, this increases the smell, not decreases it.

Really, I've been in a shelter with over 70 ferrets in a relatively
small space, and the only thing that I smelled was the wood-pellet
litter. (clean wood-pellet smell) We even had people off the street
that were amazed because they didn't smell the ferrets either.

Personally, I'd never use any of the "ferret colognes". 

[Posted in FML 5419]