Goals should never be conceded when defending a set piece. A goal given away during a set play is a cheap goal. Each player is aware of what is about to happen and if a goal is conceded it is because of a lapse in concentration or an exceptional piece of play from the opposition.
Defending a set piece is not difficult or complicated. The most important thing is to ensure each attacking player is marked. The defender must stay with their attacker and do so on the goal side, so that the defender is always closer to the goal than the attacker.
When defending a corner it is a good move to ensure you have a player on each post. The defending player closest to the corner kicker should face the ball, keeping tight to the post to prevent the ball sneaking directly in from the kick. The player on the far post should face play, anticipating the ball coming in from an attacker.
For 7v7 mini-soccer, you should always have a player on the near post, especially because corners from young players do not always reach past the near post.
The other defenders should mark each of the attackers, following their movements and keeping tight to them. It is important to keep contact with the attacker, so that you can feel as well as see where they go.
Often the attacking team will line-up a player or two to run into the box as the kicker takes the corner. It is also important to mark these players and run in with them.
If the attacking team take a short corner it is essential for a defender to quickly close the ball down, preventing the ball coming in from a better angle. However the players on the posts should remain in position until the ball is cleared.
Defending throw-ins is much like defending a corner with each attacking player being marked by a defender. When the ball is thrown down the line, where possible the likely recipient should have a defender both in front and behind. Doing this prevents and easy fet and back ball, as well as dealing with a longer ball down the line.
For deep free-kicks from a far distance or wide kicks, the same technique used to defend a corner applies. Each player should be marked at the goal side, though depending on where the ball is coming from may result in not needing a player on the post.
For free-kicks closer to goal it is important to react fast by getting as many players behind the ball as possible, preventing being caught out by the kick being taken quickly. If the free-kick is indirect, then a player must close the ball down as soon as it has been touched, and the player must be prepared to put his body in the way to block any potential shot.
To defend a direct free-kick a wall may be required, which should be organised by the defending goal-keeper. The wall should cover half the goal with the other side being taken by the goal-keeper.
Players not in the wall should each mark an attacking player.
The most important part of defending any set piece situation is to communicate with your team mates. Telling each other who you are marking, making it clear to all defenders but also informing your team mates if any players have been left unmarked.
If you pick up a player, defending on the goal side, and stick with that player you will have a very good chance of preventing a goal from a set piece.