I've taxidermyed squrril and chipmonk hides for a while and I used the soak in salt water then stretch and dry method but what I was wondering is how do your turn the hide from stiff as cardboard to a nice (flexable) hide?

You will want to use a softener and then break the gide.
VanDykes is proboly the best place for retail consumers for the softener and a break or make your own.

Once they are dry just grab them and strech them or work the over a egde of a table.

Do it the old fashion way.....
Marry an Indian Squall and let her chew on it