A player can always improve his fitness by working out hard. He can comprehend certain tactics by studying the game. But how far he or she goes will be determined mainly by how well he or she has mastered ball skills. Those are acquired by playing, day after day, year after year.
A player who really wants to excel will spend as much time as possible playing small-sided games when he has playmates, and juggling and kicking against the wall when he's on his own.
I spent a lot of time hitting the ball against the side of the house when I was a growing up. If my mother complained about the noise, I'd hop down the retaining wall at the end of our property to the office-building parking lot.
I'd use that wall -- hitting the ball with both feet, seeing how long I could return the wall's passes without losing control. I found out later that so many pros spent lots of their childhood doing that.
Dennis Bergkamp, the great Dutch striker who scored and set up hundreds of goals for Ajax Amsterdam, Arsenal, and the Dutch national team, said that when he was a youth player at Ajax, they had little three-foot-high walls. He would knock the ball against the walls for hours. Every time he hit the ball, he'd know whether it was a good touch or a bad touch. He'd do it over and over, trying to establish a rhythm.
Whenever I saw Bergkamp slotting a perfectly placed ball past a goalkeeper or making a precise pass, I thought of him practicing against the wall.
Kicking against the wall is an excellent way to work on improving your weaker foot. You can back up and practice shots on goal, or move close to the wall and work on passing, because where there's a wall, there's a teammate.
You can practice trapping and work on your first touch by controlling the ball before you kick it, or hit it back first time.
Passing the ball against a wall from close distance takes timing and coordination. Hit the ball faster, and you've got to react faster and get a rhythm going. It almost feels like you're dancing.
Practicing the correct striking of the ball over and over helps it become second nature. It has to be, because in a game a player doesn't have time to think about his form or approach. Under pressure, everything is more difficult. Mastering technique while playing on your own is the first step to being able to do it right in a game.