The Great Equalizer; Endurance
Soccer is a long game with lots of running. The team with the most hustle and endurance can sometimes make up a skill level difference of a competitor. This becomes even more important in tournament play where a team might play four games in two days. For instance, "in this nonstop game of 90 minutes or more, endurance. You need to be able to run six or more miles, sprinting, jogging and sometimes going backwards at full speed if you are a defender" says LiveStrong.com.
The best time to build your endurance is before the season starts, an continuing to build as the long season wears on.
See some Endurance Exercises here.