A good basic passing drill this can be used as part of pre-season training or to get training sessions off to a positive start before attempting more complex work on other skill sets.
Place two cones about ten yards apart on the pitch and line up a group of player behind each one. Each cone can have three to four balls alongside it, although inform your players you expect them to really only need one, and keep it in play for the duration of this session.
At the coaches whistle the team standing by the cone nominated by the coach as the start point begins, with the first player in the group passing the ball quickly and crisply along the ground to the player by the opposite cone. As soon as this first player has played his pass he moves to the left of the cones, and loops around to join the back of the opposite group.
As the pass is played, encourage the player receiving it to move towards the ball, rather than wait passively for it to arrive, and play the ball back quickly and firmly. Players should reach the ball with forward momentum, with their body correctly positioned to play the pass - standing foot alongside the ball, body weight above the ball to keep the pass from rising, and the pass played with the side of the boot (the biggest and most stable surface used to maximize control).
This drill can be modified easily - for example increasing the distance between the cones or challenging players to pass the ball through the air. If your squad can perform this drill through to completion using only one ball, allow this, otherwise emphasize the need to repeat, repeat, repeat these skills. You cannot over emphasize a mindset of close passing, and players moving actively to receive a ball in motion.
For pre-season you should also insist that players call out the name of the player they are passing to. Not only does this ensure the squad quickly become familiar with, and integrate any new players, it also ensure no matter how badly a result may go during the regular season your players can never be accused of "looking like they didn't even know each other's names".