Not sure of the best way as theres several ways to do it depending on what your working with. If its a thin skeleton a utility knife can work, if the skeleton is thicker a dremel with diamond tip as suggested is also a great way. If you want to keep the over all size but just want to trim a few wings or outgrowths you can do just that, or if the piece is sizeable cut a ring around the center part of the colony, think of a doughnut. Then with that dougnut ring cut those into 1-3" pieces, again depending on the colony your working with, theres no rule.
While cutting with a dremel it's important to keep the tissue and blade wet or it will get to hot and more tissue will recess from the cut. After the pieces have been cut place them in a bowl of tank water so that their cuts will coagulate, this also helps from all the frags polluting the tank irrating other inhabitants. After a few minutes place them near or around it's original location. This would leave you with plenty of frags to share and a nice shaped colony to grow out again.
-Justin