A letter of recommendation is an act of persuasion. It is your attempt to persuade the letter’s recipients that this particular individual is the best choice for the program or position.
By its very name, a letter of recommendation should present a positive picture of the person who is being recommended. So it is important that you agree to write such a letter only for someone about whom you can make positive statements. (One of the most difficult things to do is to inform a student or colleague that you can not write a very positive letter for him/her, but it is important to do so at the outset so that he/she can ask someone else to write the letter.) A letter that "damns with faint praise" is of no use to anyone.
As with most writing tasks, writing a letter of recommendation is made easier if you do some prewriting activities first. In this case, the prewriting involves requesting the applicant’s current resume and a complete description of the program or position that he/she is seeking. With this information in hand, put yourself in the place of the people who will receive the letter. What information would help them make an informed judgment about this applicant? What traits and skills would be needed for this position? Does the applicant have them? In short, the more information you have, the more persuasive your letter will be.
Content and Structure of the Letter
The overall goal of the letter should be to present a positive and realistic picture of the applicant, giving the letter’s readers a sense that you know this person well and that you believe the applicant has the traits, the training, the background, and/or the potential to perform well in the program or position. Avoid, however, presenting the applicant as unrealistically wonderful. A letter that "damns with exaggerated praise" is of no use to anyone.
As with most writing tasks, a letter of recommendation should be crafted to fit the situation and the audience. A letter that successfully recommends an applicant for a spot in graduate school might not be effective as a recommendation for a job in company. One of the benefits of putting yourself in the position of the letter’s readers is that you can imagine what will and what will not be persuasive to them.
Begin the letter by explaining your relationship with the applicant. Under what circumstances have you known him/her? For how long have you known him/her? Are you evaluating him/her on the basis of performance in a classroom? In a lab? As a TA or an RA? As a colleague? Then discuss the applicant’s traits or skills that are appropriate to this particular program or position.
As with most writing, specific details and examples are compelling for two reasons: first, they demonstrate the depth of your familiarity with the applicant; second, they create additional insight into the applicant.
Usually, a letter of recommendation should have three or four paragraphs and should not exceed one page in length.
The Letter and You
Part of the letter’s persuasive power comes from the image of you created by that letter (your ethos). If you are unknown to the letter’s recipients, it might be a good idea to mention your own credentials where appropriate (e.g., "In my twenty years of teaching at MIT, I have rarely had a student who …"). Standard academic prose and unambiguous language will add to the image of you as a professional who is an able communicator.