A good defensive header will help to get your team out of trouble. The main focus behind a good defensive header should be on getting height and distance on the ball. Getting the ball out of the danger area should be a priority.
Step 1: Drill 1
With three players you will need a feeder in the middle and a player either side. The feeder should throw the ball to one of the players who should the head over the feeder for the other player to catch.
Step 2: Drill 2
Continue with drill two but this time after the first header the receiver should try to head back the second header to the feeder. When you've completed five, swap positions.
Step 3: Drill 3
As an extension of drill two start with a throw from the feeder to the defender. The defender should head the ball over the feeder to the other player. The player heading the ball second should then try to play an attacking header through the feeder's legs.