Always be firm, but quick to praise. Let players know when they have done something correct. You must develop a mutual respect for one another.
Get to know the players fairly well before chewing out any of them for game-related mistakes (as opposed to disruptive behavior). Do so only after determining which ones will react positively to scolding. Some will accept criticism if it is done in a light-hearted manner. But that can only happen after a period of time during which trust has been engendered.
Guide the team leaders so that they can develop effective ways to motivate their teammates. In some cases, you can profit by observing how they interact with their peers.