This is an excerpt from Premier Soccer by Michael Parker.
Since each system has its pros and cons, it is impossible to generally recommend one over another. The choice of system depends on criteria including the types of players available, the formation used by the opponent, the tactical approach to a game, and the field conditions. What is most important is to use a system that fits the strengths of the players. A coach should get the best 11 players on the field, then find a system that meets their needs. Sometimes this requires moving a player to another position, and certain positions are easy to adapt to. For instance, most forwards can be taught fairly easily to play on the wing, and a midfield player can typically serve as a defender.
Next, the coach must consider team style. To play direct soccer and get the ball forward quickly with longer passes, it may be best to use more forwards. To focus on shorter passing through midfield, the coach might want an extra midfield player. To defend well, an extra defender might be the key. Whatever the case, the coach should play to the team's strengths, not to her own preference or comfort zone.
A coach may also consider changing a system to match up effectively with a particular opponent, but care must be taken in doing so, since teams should not regularly change how they play the game. The coach should be more concerned about preparing his own team to play to its strengths than about reacting to those of the opposition. Nevertheless, some consideration does have to be given to the opposition.
If pressed, I would say that the best system to use is generally the 4-4-2. Many coaches would disagree, but it's probably fair to say that this system places players across the field more evenly and covers the field better than any other. We have talked about team shape and its importance, and the 4-4-2 system is better suited than most to maintaining good shape.
This is an excerpt from Premier Soccer.