Go to the nearest building or vehicle during a storm.  Taking shelter from a tree only increases your chances of being struck from lightning.   Count 5 seconds per mile from when you see lightning until you hear thunder. So 10 seconds is 2 miles, etc.
Watch for developing or approaching storms.
Go inside a building-stay away from windows, pipes, and electrical outlets.
Go inside a car, truck, and avoid contact with metal.
For group activities outdoors, designate a person to track the distance to lightning. 
Stay in a safe place until the threat of lightning is over.
If caught outside far from a safe shelter, stay away from high objects, as well as fences, wires, poles, and especially trees.
Be aware of your surroundings.

Text courtesy of poster distributed by the National Severe Storms Laboratory.