Being asked to write a letter of recommendation is ego-gratifying, but at the same time is a significant responsibility. A well-written letter of recommendation can make a real difference in the chances of a candidate of any sort, and by the same token an unfocused or poorly written letter can hurt her chances. Although there are slightly different expectations for letters of recommendation in different fields, there are a few general rules to keep in mind when writing a letter of recommendation, and considering them when writing your letter will maximize the applicant's chances for acceptance.
Request a brief meeting with the person requesting the letter of recommendation unless you already know her very well. Use the meeting to get more detailed information to include in the letter as well as to get a better sense of her expectations for the letter.
Read and follow the instructions accompanying the letter of recommendation, if any. Hiring managers and award and scholarship committee members frequently find that letters of recommendation do not provide the kind of information about candidates they need to make the best decisions.
Begin the letter with a brief summary of your relationship with the applicant, including how long you have known her. Except for high-level job recommendations, it is best to be succinct in this introductory section; a couple of sentences is fine.
Use the body of the letter to describe the character of the applicant. Give specific examples of how the applicant contributed to the success of a project at work or performed research that advanced an academic field or showed great potential. While it is important to be positive and focus on the strengths of the applicant, if she needs to work on certain areas to be successful at the endeavor being applied for, brief references to areas that need improvement make the letter seem more honest, and might actually make the applicant stand out in the mind of those reading the letter of recommendation.
Close the letter with a strong statement of personal support for the applicant. Specifically mention the position or award or fellowship being applied for and that you feel the applicant is an ideal candidate. Include contact information such as an email or mailing address and mention that you will be happy to answer any additional questions.