The Principles of defence:
The most important aspect of defending is the ability to delay the attack - by shutting down the space ahead of the player with the ball - this action gives the player with the ball only two options - to attack and take you, the defender on, or to pass the ball.
If the player decides to attack you should “back away” and show the attacking player a certain channel in which YOU will allow him/her to progress - this is delay - holding up the advance of the player so that other defenders can get behind the ball.
Don’t commit yourself to a tackle unless - there is a another covering player behind you - or - you are absolutely sure that you can win the ball.
When confronting the player with the ball - don’t get square - approaching side-on so that if he changes direction - you - the defender can - back away - saving yourself time.
TEAM WORK: In the modern game - most teams will play with just two attacking players - these are arguably their best players. It is important to identify these players and then delegate which one of the defenders will mark them. Usually the two middle defenders will pick up the opposing players while the outside defenders will pick up wide players.

Stay in contact with your goalkeeper
COMMUNICATION - It is vital to communicate with your team-mates around you - keep working as a team.
CONCENTRATION - when confronted by an attacker with the ball - watch the ball - never the player.
BE AWARE - Remember to take a quick glance around you when the attacking team in advancing towards you.
STAYING GOAL-SIDE: Put yourself in between the attacking players and the goal - making a shield - remember you are defending your goal.
CLOSING DOWN - Never allow a striker a direct line to goal - by closing down - getting in front of the attacker you are not allowing a shot at goal.
OFFSIDE - an effective and powerful defensive strategy: if executed correctly will not only kill the offensive momentum of the opposing side but will give you possession of the ball.

Don't commit - unless your 100% sure of winning the ball
One Defensive player should be ultimately responsible for calling the defensive players out - the defensive captain - knowing the exact moment when to spring the trap is vital and when it is called - there MUST BE NO hesitation.