1) recognize that your everyday conversations are a primary source for growing vocabulary. Talk with your children throughout the day, during read aloud time, at lunch, outside. No preschool teacher should be at her desk or standing to the side during center or free play time.
Get down on the children's level and inquire, investigate, draw in those words from your read aloud that may be new to your children. When you teach on themes, this is easy. Have a new word to celebrate each day or each week and use it often, in different circumstances and contexts.
Don't be afraid to use big words with them. But use them as naturally as you are able. Even if you aren't used to talking that way, be purposeful in your use of those words with them. Connect them to synonyms that your children already know. I love Ogden Nash's Adventures of Isabelle because in the midst of teaching children to be confident, he uses great words like "cavernous" and "ravenous". Here are a few other titles that are excellent for growing vocabulary (did you know you can do that, even if there are few or no words?). You'll be able to find a complete list of such titles to build conversation in my upcoming book for teachers, Before They Read: Language and Literacy Development Through Conversations, Interactive Read-alouds and Listening Games (to be released in November, 2009). Here's a taste, but stay tuned!
Actual Size by Steve Jenkins
Bear Snores On (and other books by Karma Wilson)
Dear Zoo: A Lift-the-Flap Book by Rod Campbell
Fuzzy, Fuzzy, Fuzzy by Sarah Boynton
I Like Black and White by Barbara Jean Hicks
2) Share those same words with your families that you use during the day so they can talk about those big (and not so big) interesting words too and use them in conversations with their children. Do you know what "decontextualized" language is? That's speaking in the future or past tense, rather than just the here and now. Talking in that way not only encourages vocabulary but also more abstract thinking skills. You may already do that when you talk in circle time about what the children had for breakfast or plan together what you will do later in the day. Be sure to remember to send those books with great vocabulary home so the children can explore them with Mom or Dad, big brother, Grandma or a neighbor.
Oral language is the basis for written language. When your preschool children are ready to learn to read conventionally (either before or after they go to kindergarten -- either might be normal for them), their strong vocabulary will make reading easier. They'll listen to the word as they sound it out and blend it, it connects to that word they know in their oral language and BAM! they've got it. Ask thinking questions to expand their language; use scientific terms like "document" instead of "write down" or "experiment" instead of "try it out." With a little thought, you'll have a great impact!