In a scrimmage with my 07 team Saturday, it was our first time playing 9v9, and it was also the first time we played with 2 centerbacks. An issue we had in the first half was that they were consistently too far behind the midfielders when we lost the ball in the other team's half. This allowed the opposing forwards time to receive the ball and run at our back line. Neither of my centerbacks are particularly quick, and this was a huge problem for us.
In the second half it improved, and I emphasized that the way we want to defend is by stepping up towards the ball, rather than dropping back to the goal. Several times in the second half, their forward would receive a pass to his feet, but this time the centerbacks were able to apply pressure right away and either steal the ball, or affect the attacker enough that we could get numbers back to defend.
The key to defending is to minimize the amount of space you need to cover. This goes for both high pressure or more passive approaches. However you plan to defend, it's important that everyone is on the same page, and is moving to defend as a unit. Take, for example, these 2 squares:
By shrinking each side by only 1 foot, the total area contained by the square is reduced by nearly 25%. If you imagine that there are 4 defenders within each square, in the first square they're responsible for about 16 square feet each. In the second, they are responsible for about 12 square feet.
The above abstraction can be directly applied in a coaching context. If you get every player to shrink a few yards closer together, it will have a tremendous impact on your defending. This is where the phrase 'greater than the sum of its parts' comes into effect. If your team turns the ball over in midfield, and the back line steps forwards to pressure while the mids drop back to defend, the amount of space the opposing team has to operate in will be dramatically reduced.
This doesn't apply just to a vertical axis, but horizontally as well. If the other team has the ball on your right wing, your left-sided players need to be shifting over.
My centerbacks didn't become more skilled defenders at half time, but by reducing the amount of space they had to cover, the were more consistently halting attacks.