Designate a "channel" on each side of the field.
One player from each team is allowed to go into the channel.
If the ball is played into the channel, the player can dribble, without being defended, and serve the ball into the box.
Progress by allowing the "server" to defend in the channel, but, at the same time, allow a teammate to support inside the channel as well.
Proper technique of crossing
Timing of runs into the box.
Defensive heading: Head high, far and wide.
Attacking heading: head down on goal.