Encourage pretend play. Pretend play encourages preschoolers to use their vocabulary.
Read older books and fairy tales that might contain unfamiliar language and concepts to your preschooler. (In Cinderella, your preschooler might see pictures of a coal stove or the pumpkin "coach." These objects are not part of our everyday life.)
Use different words for objects. Instead of saying "car," say "automobile." By doing this, your preschooler will start to recognize that different words can define the same thing.