There a lots of ways to teach voice. I like to begin by having each student create their own "journal". They learn that they all have a different voice and when the journals all look different they can't wait to write in them. My kids say, "I'd know it was **** by the tone in the paper!" Also, the voice needs to mathc the audience. Have you heard of RAFT? It is a great way to get kids to think about who they are addressing and the "tone" or "voice" they'll need to use. Then, we read books that have strong "voice" like:
Two Bad Ants
or Horrible Harry and the Horrible Rotten No Good Day
Look up lesson on line, or pick up one of the great resources we are all sharing on the favorite resources post. If lots of us suggest them, they are probably worth your time!
Here is a site with books listed by trait: READ them you kids!! Talk traits all the time!!!