What I have heard works good is:
- use sharp scissors and cut the head off as far down the base as possible
- cut into pie shaped pieces - best if you can get a piece of the mouth in each frag
- put in a container with rubble/crushed coral in a low flow area for a week or so until they attach to something
I have a couple of shrooms on a small piece of live rock - I just tried this a couple of weeks ago to test - just took out the rock and used a sharp knife to slice the shrooms in 1/2 (through the mouth). Doesn't make 'frags', but does double the number of shrooms. In my tank, takes about a week or so to fully heal. I've only tried 2 shrooms twice, but it has worked both times.
Let us know how it goes! I might try that first technique tonight for fun on some different ones.