a few suggestions:
-point a little bit of flow at it. they will try and move out of the way and leave little babies behind.
-slice it right down the center. it will eventually close up into 2 shrooms.
-if the mushroom is large enough, slice it into quarters.
-throw a bunch into a food processor, chop up a bit, and then throw them into your tank. you won't have control of where they end up, but you will have a lot of them in a month or two.