At the third grade level, we teach the kids the acronym PIE for persuade, inform, and entertain. We chart this and some examples either of stories/books we have read that fit into those categories, or maybe something like a commerical from TV or an editorial that would have one of the purposes listed. Then, you could try a lesson like this one, once the students have mastered the types of purposes:
www.newton.mec.edu/angier/DimSum/ W.W.%20Auth's%20Purpose%20Lesson.html - 7k -
I also liked this site for closure. It allows the children to interact with the computer to select the right purpose.
http://www.rcs.k12.tn.us/rcs_web/Title_I/author's_purpose_scavenger_hunt.htm
I hope these help you out.