1) Coaches should stay safe. Let's remember that players are changing physically at this age. Many have little experience with the game you are coaching. Most adolescents regularly do not participate in vigorous physical activity. Let's assess the physical abilities of each player before pushing them hard physically. Find out where they are and work up from there. Obviously players need some degree of physical conditioning to play, but don't set the bar really high and find out who can perform at that level. Players get freaked out and hurt by that approach. In other words, we should not come into tryouts assuming they are ready physically to compete in a game situation. Gradually prepare them physically for the first game. Make sure your athletic director schedules your first game at least three weeks after tryouts!
2) Coaches should develop well-rounded players rather than positional players. In other words, coaches should emphasize the fundamentals (knowledge, skills, strategies, physical requirements). Middle school really is the beginning of adolescence - the period of growth from childhood to adulthood - so athletics in this period should be fairly simple and straightforward. Help players understand the basics of every position. Players will need that knowledge to succeed. Don't be surprised when you need to explain something as basic as a "throw in" for soccer, "traveling" for basketball, or the number of outs in an inning (I have seen confused middle school players not sure what to do in each of these situations). Many players have played only in recreation leagues. Let's assume players know very little about the game and help them develop the necessary skills to enjoy the game for a lifetime. Make a list of basic terms you want to teach to your team throughout the season. At this level knowing the rules of the game can give your team a decided advantage in a close game.
3) Coaches should encourage players to explore and test different talents and positions. For example, if a player shows interest or signs of talent, let them try goalie or point guard. Generally I think we should avoid leaning toward specialization at this age. Simultaneously I think coaches can play someone primarily at one position, especially for critical positions. (e.g. goalie, point guard, catcher). However I still think coaches should challenge players and parents to try other positions as well. The style and sophistication of varsity athletics comes with experience and growth.
4) Coaches should help players take risks rather than avoid failure. This implication ties in with the previous one. We should obviously set up players for success and avoid putting them in situations where they will fail miserably. However, let's encourage them to take risks and test something (including themselves). As Christians, this implications sets up many opportunities to trust God, rely on his strength, and grow as courageous men and women.
5) Coaches should play everyone. Obviously players will have different levels of talent and because middle school sports is competitive, everyone should not play equally. However, players need experience in games in order to grow more comfortable and confident in competitive situations. They need pressure in order to grow comfortable with pressure.
6) Coaches should have fun with their players. Having fun should be a constant theme in sports at every possible level because sports is playing a game. Emphasizing fun at this age helps players grow more comfortable with the competitive elements of athletics; it also helps us to not take ourselves so seriously.