1: Keep POSITIVE support, encouragement, cheer leading and general screaming and hollering to a MAXIMUM on the touchlines.
When the players are working hard, they need and deserve everyone's best POSITIVE encouragement and support. They need to know you're there and that their effort is appreciated. Most teams have a tough enough time developing a sense of teamwork and achievement at the same time the players are gaining experience and skill. They DO NOT need to hear YOUR anxiety piled on top of their own when the game is going poorly.
2: DO NOT CRITICIZE referees or players from either team for any reason.
If the referees really ARE doing poorly, they may get angry or offended by critical spectators and that may make things tougher for the team. If they are good at what they do, they will ignore you, or perhaps ask you to leave the field. Either situation is at best distracting and at worst reflects poorly on the team's overall sportsmanship. If there is an issue you bring it up with the board and not the fans, coach, ref or players.
3: Don't coach players from the touchlines, or for that matter while THEY are on the touchlines.
In most leagues, coaching from the sidelines is frowned on, and rightly so. Soccer is different from most sports in the US, because it is a game of the players. Coaches are supposed to intrude as little as possible. If you feel a child is not doing what should be done, tell the coaches, not the player. As parents occasionally discover, a player may be doing EXACTLY what the coaches have instructed. Either way, a parent can help a player's athletic development much better working together with the coaches, not independently.
4: Give the players, coaches and referees room to work.
Many organizations have rules which require that spectators on the touchlines stay in an area between the penalty boxes, and keep all parts of their bodies (even outstretched feet) at least 10 feet behind the touchline. Do not crowd the touchlines for any reason and stay away from the goal area to avoid interfering with those involved in the game. If necessary, gameplay will be halted until these areas are cleared. *NOTE: ONLY PLAYERS ARE ALLOWED IN BETWEEN FIELDS AND ON THE BENCHES! Please call for the Team Mom or one of the coaches if you need to speak with your child or give/get something to them.
5: Remember, IT'S ONLY A GAME.
Don't forget, YOUR attitude on the touchlines can affect the mood and success of the team. If the coaches think that your touchlines activity is hurting team performance in any way, they should promptly advise you, hopefully without ruffling any of your feathers. Be tolerant. Emotions run high during games, and feelings are easily hurt. Nevertheless, any spectator, whether parent, friend or player, who persists in inappropriate touchlines behavior after being warned by the coaches should be asked to leave the vicinity of the field. Coaches should not argue with parents. Bring all issues up with a board member so they can handle it appropriately.