Provide preschool age children with opportunities for turn taking as much as possible. At this age, sharing is not the most desired skill a child wants to learn. You can help combat the "me firsts" by practicing turn taking. Play games like Candyland and Memory during small group times that require children wait for their turn. Make the groups small so the wait time is low. Young children don't want to wait long for a turn. As the school year progresses, play larger group games, like Hot Potato and Duck, Duck, Goose, that offer longer times between turns.
Pretend play gives children a chance to step out of themselves and be someone else for a while. Provide them with pretend play items that are uncommon to them to build imagination and self confidence. Most preschool classrooms offer an area to work in the kitchen and play house. Provide pretend toys to set up a restaurant and assign a cook, a hostess and servers. You can create a vet's or doctor's office and assign roles. Give kids a chance to create outside of their normal scope to build self esteem and independence as they work together.
Give children a chance to have ownership of the classroom by providing classroom jobs. This builds self confidence as they are successful at helping and it also builds community. Children want to be involved in all parts of a classroom. Let them take care of tasks like feeding classroom pets, passing out books and passing out snacks. They can also take turns being a line leader or lunch helper. This gives them a sense of responsibility. This also reinforces the skill of turn taking as the children will have to take turns doing jobs.
Offer children a chance to help make classroom rules. If they have an active role in making rules, they will have more investment in following them. Guide them in making rules like being safe, kind and responsible. Have them give examples of how to be these things and practice them as a group. Make a poster that lists each rule and has pictures of the preschoolers following the rules. Seeing themselves on the poster will give them all the more incentive to behave. Go over the rule poster daily to reinforce the social niceties expected in the classroom.