I teach 3rd grade and use a fun little way to teach/intro Author's Purpose. We have a tv in our classroom. So, I flip it on and have a list of channels already written down on a piece of paper. After we've discussed the meaning of the main trypes of writing, I tunr to those channels and they must respond with the correct answer. For example, if I turn the tv to CNN, they will say "Inform." If I turn to NICKELODEON, they'll say to "Entertain." We have the TV on, but they hardly realize that we're using it as a learning tool! Worked well for 3 years now!
Hope this helps!