Employ an age-appropriate curriculum. If you aren't a curriculum writer, and don't have experience with writing curriculum, purchase or find preschool curriculum that has been written expressly for the 3-to-5 age group.
Create a classroom environment that is appropriate for preschool students. Desks, chairs, cubbies and all other furniture should be child-sized. Posters and pictures on the walls should have big printing and be mostly pictures. Books should be appropriate for ages 3 to 5, and you should also have a large amount of learning objects, like letter and word cards, number blocks, and games meant for preschool students.
Create instruction based around several learning styles. Talk to students, and give them visual aids. Allow students to participate to learn, and demonstrate for them as well. With young children it is important to teach in several different ways so the information has the greatest chance of being understood.
Create short lessons and include hands-on activities so students don't get bored and so the largest amount of learning can take place.