The first topic I would like to cover is a player's mentality during
training and in games.  I have seen many practices where players are
lagging throughout training.  Not giving 100% effort.  Then they expect
to play in a game and give that 100% effort that they have never
trained to do.  The mentality that players need to develop is one of
constant intensity.  For example, players should train every second of
every practice as if the clock is winding down in an Olympic final. 
Not a second wasted, not an ounce of energy saved.  It's all left on
the field.  This has to be done consistently in training in order to
get the true effect in games.  Many players train at 50% thinking they
can raise it to 100% in games.  This is not a switch you can turn on
and off.  How you play in training is how you will play in games.
   

One way to improve your training mentality is to play to win.  Don't
allow yourself to lose, whether its 1v1, or a game related situation. 
Do whatever you can to win.  Every position requires a certain
mentality.  Forwards need to know its ok to take risks, because it's
all about wearing down your defender.  Midfielders should seek and win
duels, challenge for head balls, win tackles and come up with loose
balls.  Defenders should win 1v1 duels, outlast their attacker, take
pride in your defense and refuse to let your opponents get by you or
get a shot on goal.  Remember, the more intense you are, the better you
will perform in games, and the better you will make your teammates. 
Odds are you are not training nearly as intense as you could.  Try to
make a change.  However, you must understand that how you train makes a
difference in how you will play.