Below are a few defensive concepts that I believe should be a part of you playbook if you want to defend well as a team.
The first rule of defense should be dictation. Defense should dictate to the offense and not let the offense control the game. Basketball is a "read and react" game. In most cases, the offense makes a move and the defense reacts and tries to stop it. Good defensive team forces the offense to react to them.
When the offense brings the ball up the floor, does your team stand back and let them decide where they are going to start? Or do the force the ball to a particular area or to a particular player? When faced with a ball screen, do you force the ball to or away from the screen? When playing cutters, do you allow them to come through or do you try to force them to take a different route. When playing cross screens or down screens, do you allow the action to take place or do you force adjustments?
Forcing the offense to play your style and in the areas where you are strongest is the first step in becoming a better defensive team.
Shrink The Court
It stands to reason that the smaller the area you have to defend, the better your defense is going to be. Cutting off the ring has long been a successful tactic in boxing when in a bout with a very mobile opponent. Give your team areas that you want the ball to be in and don't let it out. My personal preference is to force the offense to play on only one side of the floor. By keeping the ball on one side of the floor, you can limit penetration, reach shooter, even determine who will handle the ball. If you play at a level where you play with a shot clock, not allowing the ball to reverse is especially effective.
Other coaches try to force the ball to the baseline, others to the corner, etc. Though the areas are different the thoughts are the same. Force the offense to play in only one area.
It stands to reason that if you can have more defenders than offensive players in an area, the more effective the defense will be. That is the concept of help. When one defender gets beat, another is there to take up the slack. That is defensive help. Defensive help gets better when you are able to dictate to the ball and shrink the court; your defensive help improves markedly.
What do your players do when they are not guarding the ball? Do they stand and watch? Do they play their man? We like to teach, when on ball side, you guard the man first and the ball second. We also teach that the man with the ball is the most dangerous player on the court. When the ball is on the dribble and going to the basket, it is everyone's job to guard the ball. Help defense is critical to stopping the penetration, layups and the problems that they cause. If your help can keep the ball out of the lane, your defense will be much more effective.
Talking is the key to drawing any group together is talking. Can you imagine sitting in a room with a group of people with a common goal and not talking to one another? How will you accomplish your goal?
Players have not problem talking on offense. Simple things like "I'm open," or "Throw me the ball!" seem to get players involved with one another. Defensively, simple things like, "screen right," "I got him left," "drive him my way," or "I've got your help right," etc., will get your team to play as one. Once everyone knows what their teammates are doing and they are behind one another, confidence goes way up as does performance.
If you want your team to player better defense don't just practice defense, you have to practice team defense. Build your defense so your players can help one another and your team defense gets better.