The correct technique involves using the frontal bone just below the hairline (the hardest part of the skull) while simultaneously isometrically contracting the neck musculature.
The soccer player should then apply a counterforce to the impacting ball by flexing at the trunk.
By performing this action as described, the athlete's body becomes a single, rigid unit that lowers the risk of injury by decreasing the rotational or linear forces to the head as forces generated by the ball are dispersed across the athlete's body.
Keep in Mind: Many concussions are the result of a soccer ball colliding with the head of an unsuspecting soccer player. The neck musculature does not maintain the head in a rigid position at ball inmpact, forcing the cranium into the brain and resulting in a coup injury. If the brain then rebounds into the opposite side of the inner cranium, a contrecoup injury can result.
This is demonstrated in this video by A.D.A.M: http://www.youtube.com/watch?v=fY7J7bccNoU