Ceiling fans should run counterclockwise during the summer to force air downwards, creating a cool breeze.
Ceiling fans can be run clockwise in the summer so that the cool air is not blown directly on people in the room. The fan will still cool the air, but it will not send the breeze downward.
If your ceilings are higher than 8 feet, run the ceiling fan on high speed so the air is forced down low enough to cool the room during the summer.