I literally slice them in half and make sure the mouth is split evenly. They heal almost immediately, if they're still too big, I wait a week or so and do it again.
Caution: THEY SMELL WHEN YOU FRAG THEM!
Take the rock out that they're on, put in a bucket with some water and use a razor blade, after you frag, just gently swish them around in the water a little because they tend to "bleed" and then they can go right back in the tank. I have to frag mine about once every 2 or 3 months.