An effective letter of recommendation:
1. Explains how you know the student. What is the context for your evaluation? Was the student in your class, an advisee, a research assistant?
2. Evaluates the student within your area of knowledge. Within the context in which you know the student, how did he or she perform? How effective a research assistant is he or she?
3. Evaluates the student's academic capacity. This is easy if the student was in your class. What if the student is not? You can refer to his or her transcript, but only very briefly as the committee will have a copy. Don't waste space talking about objective material they already have. Talk about your experience with the student. If a research assistant you should have some grasp on his or her academic competence. If an advisee, refer briefly to your discussions and provide clear examples that illustrate academic potential. If you have little as academic contact with the student, then make a broad evaluative statement and use evidence from another area to support. For example, I expect Stu Dent to be a meticulous student, as he keeps very careful and accurate records as the Biology Club Treasurer.
4. Evaluates the student's motivation. Graduate study entails more than academic skills. It's a long haul that takes a great deal of perseverance.
5. Evaluates the student's maturity and psychological competence. Is the student mature enough to accept the responsibility and manage the inevitable criticisms and even failures that will accompany graduate study.
6. Discusses the student's strengths. What are his or her most positive attributes? Provide examples to illustrate.
7. Is detailed. One of the most important things you can do in improving the effectiveness f your letter is to make it as detailed as possible. Don't just tell them abut the student, show them. Don't just say that the student can understand complex topics or work well with others, provide detailed examples that illustrate your point.
8. Is honest. Remember that although you want student to get in to graduate school, it is your name that is on the line. If the student really isn't a good fit for graduate study and you recommend him anyway, the faculty at that school potentially could remember and in the future take your letters less seriously. All in all, a good letter is highly positive and detailed. Remember that a neutral letter will not help your student. Recommendation letters, in general, are very positive. Because of that, neutral letters are viewed as negative letters. If you can't writing a glowing letter of recommendation, then the most honest thing that you can do for your student is to tell him or her and decline their request to write a letter.