Writing an essay or letter of recommendation is a significant responsibility. Your support can help a student gain access to important opportunities. However, recommending an applicant who does not merit that support takes the opportunity away from others, damages your credibility and potentially gives the student false confidence in her abilities. Consider helping the applicant find a more appropriate reference if you are not confident in her ability or you don't know her well enough to write an honest appraisal.
Meet with the person who needs the letter of recommendation. Determine what he is applying for, graduate school, a job, or a fellowship, and what qualifications are required for the position he seeks. Consider his skills relative to his desired position in determining your level of support.
Structure the letter like an essay. This helps you effectively organize your thoughts. Your "thesis" is why the applicant is a strong candidate, the body consists of several supporting examples and your conclusion clearly states your recommendation. It also communicates the student's strengths in the style and format expected by most readers.
Include information about how long you've known the applicant, and in what context, in the introduction. Identify yourself as a personal reference writing a character reference, if your experience with her does not directly relate to the position she seeks. For example, if you are her boss, you can speak to her trustworthiness and responsibility, but not her ability as a chemist. Consider yourself an academic reference giving a scholarly recommendation if you have had her in several relevant classes or labs.
Write two to three paragraphs explaining the applicant's strengths. Focus on qualities that demonstrate his distinctiveness and match the requirements of the program or job. Back your claims up with specific examples, such as a high level of creativity in the clinic you supervised, for a student applying to a language degree program. Each paragraph should consist of a different example.
Conclude with a clear statement of your recommendation. Most institutions expect it to follow a predictable format. That is: "strongly recommend" when you are very confident in the applicant's ability; "recommend" when you are reasonably confident; "recommend with reservations" means you have some specific concerns; and "do not recommend." Discuss the need to write "recommend with reservations" or "do not recommend" with the applicant prior to writing the letter, if that is your position. "Recommend with reservations" may be reasonable with further explanation, such as frequent illnesses, but you should discuss it prior to submission.