Have kids stand in a large circle with the coach in the center. Bounce the ball on the ground hard mid-way between the coach and the player. Player will "head" the ball back to you. For shy players, let them watch everyone else on the team go 'round once. If they are still afraid, then gently "tap" them on the head with the ball from about 6" above the head. Encourage kids to move towards the ball and "get under it" if it's aimed a bit to the left or right of their head. As players get older and technique improves, offer an extra "point" or incentive once they are able to head the ball directly back to you.