Here are a couple of methods for preserving Coprinus comatus submitted by John Rahart.

Method #1

Clean Coprinus by suspending in water.
Place in steamer basket - steam for three minutes - let steamer water cool.
Dip in cold water and drain well.
Place in freezer container and cover with cooled steamer water about ½" from top and place in freezer.

Method #2 (a variation of above)

Clean Coprinus by suspending in water.
Place in boiling water - then let water cool.
Dip in cold water and drain well.
Place in freezer container and cover with re-cooled boil water about ½" from top and place in freezer.