First, be certain that you are adequately covering proper defensive principles with your players. That is, break things down to the fundamental level. Start with the role of the first defender. Demonstrate the proper defensive stance, demonstrate and practice "gliding-in" to close down the attacker, teach how the touchline is our friend as defenders. Practice shepherding technique, forcing attackers out wide and keeping the attacker against the touchline. While shepherding teach the players to look for opportunities to poke the ball away. Help the players understand that that the touchline is an additional defender in limiting the attacker's angle and options to get the ball inside.
Teach proper body mechanics when closing attackers down. When the attacker is on your left, left foot back, attacker on your right, right foot back, this aids in directing the attacker to less dangerous areas (wide). Teach your player's that at a fundamental level their defensive responsibilities revolve around preventing the three P's. Preventing the attacker's turn, preventing the attacker's pass, and preventing the attacker's shot. Finally, in your own penalty area, as a defender "no foul". Goalies should be reminding their defenders of this when the ball is deep in their defensive third with the commands "no turn", "no pass", "no shot", and finally "no foul".
Teach each player that they must win every ball and pressure every ball in their area! Once the player's understand these fundamental defensive techniques they will realize that they don't need brute aggressive body contact to properly defend, but can defend at a high level by performing these techniques.
In your nine step practice routine choose as a theme one of the above mentioned techniques. Remember in step three, the 1 + 1, this is where we coach the cooperative concepts. We want defensive success here; attackers need to be passive while the players learn successful fundamental defensive skills. As the coach, correct poor technique before progressing. In step four, 1 v 1, encourage the players to incorporate the defensive fundamentals as they try to stop the attacker from scoring. During step six, the small sided games whether cooperative or competitive, make it a defensive game. Keep in mind though that when were are in a competitive mode (vs) the (vs) means verbally/visually silent, at this point the players need to learn on their own through competition. Finally, in step nine (end practice) provide a moment to summarize the
practice theme by mentioning the salient defensive points. Be sure to end with positive comments regarding the session, encourage your players to continue to be good students, and to be safe in all aspects of their lives.
As the player's progress and have adequately grasped the fundamental defensive concepts, introduce the roles of the second and third defender.
I believe you'll find that the lack of "aggression" you've observed is actually improper fundamental technique. Through practice, demonstration, and positive re-enforcement, the players will show that they can defend quite well. As you'll probably remember, when you were performing the "hitting drills" during your American football practices the coach probably demonstrated the proper technique then had the team practice it often. Through proper technique and practice you learned that it wasn't that bad and this helped you get through the mental barrier…Good luck!
