Avoid letters that only tell what grade was given in what course. They are useless. Letter-writers should provide substantial information about classroom experiences. The fact that a student did the reading for the class should not be included in the recommendation. Be honest, but cautious about criticism.
Committees take it seriously.
Be fair to both the candidate and to the reader. Do not include general platitudes that the Truman scholar is a great person and is a candidate for sainthood. What the student did in high school is not relevant in the vast majority of cases. Think outside the box of tenured professors at the university. Not that those are bad, but they may not be the people who are most familiar with the candidate.