Give them a say. Children feel good when they can come up with their own solutions for sharing. In Hall's classes, she lets children role-play sharing a doll or a ball so they can practice problem-solving on their own. Or if your son's riding the lone tricycle and a younger sibling cries for a turn, guide them to a solution by asking: "How long will you be on the bike?" or "Do you think you can play with a ball while you're waiting for your turn?"
Use a timer. Randi Rose relies on timers to keep the peace in her classroom in Wellesley, MA. A sand timer or clock with moving hands lets children see the passage of time -- and there's no arguing when time's up.
Enlist unlikely helpers. When one of Annabel Olah's students is reluctant to share, the South Riding, VA, teacher pulls her aside at an appropriate time and asks her to help her show the others how to share. The child feels valued as a leader. At home, you can even appoint a younger sibling to do this task; she'll take pride guiding her older siblings.
Double up. Preschool teachers buy items like paints and balls in multiples so children don't always have to share. Think about whether a second set of crayons or a duplicate fought-over toy can reduce needless friction.