The defender is one who cannot afford to take risks when attempting to win the ball. This is because one small mistake could result in a goal for the opponent, especially if the culprit is the last man in defence. Thus, a defender's main objective is to keep the opponent in front of you and prevent telling passes or penetrating dribbles from being made.
Always position yourself goal-side, between the opponent you are marking and your goal post. This shuts off the most direct route to goal for the attacker and gives you a good view at all times.
Close the distance between yourself and the opponent when he is about to receive the ball. slow down as you reach him to maintain your balance and control.
Assume a slight crouched position with your knees flexed. From this position, somewhat like the goalkeeper's ready position, you can easily move in any direction. Position your legs in a staggered stance with one leg ahead of the other, which saves you from being nutmegged.
Try to prevent the opponent from turning with the ball to face your goal, as he can pass or dribble on.
When you feel that you have allowed the opponent with the ball to get too far away, get in close, then tackle hard to gain position of the ball. Do not overcommit in the effort as you may be beaten on the dribble.