Good information about making a split in the most recent e-newsletter from Dadant:
The Northern states can look at splitting strong colonies to help prevent swarming. If the colony has queen cells, it can be divided into two colonies. You can make the divide at midday. The number of bees will be reduced in the hive due to foragers being in the field. You can make the divide by transferring the old queen and approximately 1/2 of the brood and food to a new hive. Do not transfer any frames with queen cells on them to the new hive. You will want to move the new hive to a different location, preferably some distance from the old hive. Put new frames in the old hive to replace the frames you removed. You will have the option to requeen the old hive or, having left some queen cells, let the bees raise a new queen. By leaving the old hive in the same location, foragers will return to the old hive, which will keep the adult population at a high level.