I take mine out of the water, cut them, then put them back in a small container of water til I mount them. After I mount them I rinse them another SW cup and put them in the tank on my frag rack.
There was a link a few days ago by Dean. do a search.
The main key is to minimize further stress that your already putting things through by fragging. Try not to get the pieces too dry as your fragging and make sure the temp in the holding buckets don;t drop too low. You can frag from inside your tank btw if the mother colonies are encrusted.