I have offered some of thoughts on this subject here, but you should consider these thoughts alongside Lisa Clark Diller's recent post at the blog of The Historical Society, "'Tis the Season for Letters of Recommendation." Diller offers helpful tips for those of us who write letters on behalf of our students.
1."Be sure you want to recommend this person for this program of study."
2. "Know the student's experiences, academic and otherwise, outside of their interactions with the recommender."
3. "Be specific about their academic skils. I think it helps what papers/reading they have done for the recommender."
4. "Talk about their disposition for graduate work."
5. "Connect their experiences and interests with the programs they are applying for."
Read the entire post to see how Diller's develops these points.