How do I configure my duct dampers for summer and winter?
Dampers are used to control which areas get more airflow. In the summer, you want more (cold) air going to the upper stories and less to the lower stories. This is due to the fact that warm air rises, and so the upper stories will naturally be warmer than the lower. In the winter, this is reversed, so you want more (warm) air going to the lower stories and less going to the upper.

How much to open or close the dampers is probably going to be a personal preference as well as depend on the layout of the house and ductwork, so you may need to experiment a bit.