Effective recommendation letters explain, at minimum:
1) the context in which the student is known
2) the evaluation
3) data to support the evaluation. A letter that does not provide details to support statements about the candidate is not helpful.
[City, State, Zip Code]
Dear [Mr./Ms. Last Name]:
I am writing to you in support of [Mr./Ms. Full Name] and [his/her] desire to attend [University Name] for the [Program Title] program. Though many students ask me to make this request on their behalf, I only recommend students whom I feel are well-suited for the program of their choice. [Mr./Ms. Last Name] is one of those students and therefore, I highly recommend that [he/she] be given the opportunity to attend your university.
As Professor of Biology at University Name, I work with many students who have substantial knowledge of biology. Each year I notice that only a few outstanding students offer a unique perspective and really embrace their learning of the subject matter. StudentName has consistently shown such a strong desire to learn that I simply could not turn down [his/her] request for recommendation.
I first met StudentName in my [Course Title] course during the [Season and Year] semester. Compared to the class average of [Class Average], [Mr./Ms. Last Name] earned a [Grade] in the class. [Mr./Ms. Last Name] was evaluated on [explain basis for grades, e.g., exams, papers, etc.], in which [he/she] performed exceptionally well.
Though StudentName has consistently exceeded in all areas of [his/her] coursework, the best example of [his/her] intelligence shone through a [paper/presentation/project/etc.] on [work's title]. The work clearly showed [his/her] ability to deliver a clear, concise and well-thought presentation with a new perspective by demonstrating [embellish here].
StudentName continues to impress me with [his/her] knowledge, skill and dedication to [his/her] work. Im sure you will find [him/her] to be a student whose talents will only shine further through your graduate program. It is my hope that you will accept [his/her] admission to your university. Feel free to contact me if you require further information.