Some books will tell you that during games you should let the players play & not give instructions. That may work for older or select teams, but it isn't very practical for youth recreational teams which only practice once a week. Most leagues allow coaching from the sidelines (although sometimes only by one coach who must stay in a designated area). If the objectives are to have fun & to teach the boys and girls how to play, then coaching during the game can help achieve those objectives. There are many things that you can teach in a game that are difficult to teach in practice, especially if you only practice one time a week (a "shifting & sagging" defense is one). I look at games as another teaching opportunity. In fact, if your League will allow it, use 2 coaches during the game, one for offense & one for defense to teach your players how to "shift & sag" & to help them learn positions. This is hugely beneficial because one coach can't watch both ends of the field at the same time. Be sure to not get in the other team's way & remember you have to coach from the side lines, not the "end zones". To be courteous, you might ask the other coach if it is okay with him). Do I yell? Yes, I find it necessary to yell instructions to the players so they can hear me across the field. I don't yell negative or general comments such as: "You guys stink" or "Hustle". I yell specific instructions such as "John, push up", or "Matt, cover the center" or "Don't get thrown over" (or "punted over" or "goal kicked over"), or "Mark up behind a man" (on the other teams throw-ins, goal kicks, & free kicks) or, on the other teams corner kicks, "Mark a man goalside". I try not to show frustration or irritation & try to not single out anyone for criticism unless they aren't hustling & then I will ask "John, are you sick?" If he says "No", I ask "Are you tired?" If he says "No", I say "Then hustle". However, I do make coaching comments to correct errors. For example, if a player's passes are coming off the ground, I will say "Matt, strike the ball higher". Or, if they turn over a throw-in because their foot came off the ground I will say "Patrick, drag your toe".