Children pick up on the language and behavior of adults. Monitor the way you use language in the classroom. Be inclusive of both boys and girls by using both gender pronouns. Most importantly, when children ask uncomfortable questions about race, ability or sexuality, answer them honestly and directly -- on a preschooler's level, of course. Ducking the question can tell children that these sorts of questions are inappropriate, leading them to draw their own conclusions.
It is also incumbent upon teachers to praise students of both genders equally for the same sorts of behaviors and activities. Make sure, for instance, you don't praise only boys for sitting quietly, as this can reinforce the stereotype that boys are expected to be disruptive and full of energy. Praise girls and boys equally for good behavior, for asking intelligent questions and for reading or writing skills.
One way that children can learn stereotypes, particularly gender stereotypes, is during preschool playtime. Overcome stereotypes by making sure that students of both genders participate equally in activities that are traditionally associated with one gender or the other. Encourage both boys and girls to play with blocks, trucks and other conventionally masculine activities. Have both boys and girls do crafts, play dress-up, play house and other traditionally feminine activities.
Young children learn stereotypes not only from their teachers but from the books they read as well. Expose your students to books that tackle stereotypes by featuring characters in nontraditional roles. Try Mary Hoffman's "Amazing Grace" or Bernard Waber's "Ira Sleeps Over." Make sure to choose books with protagonists who are not only white and male. Find children's books with heroes that are of other races, female or disabled, for example.