Agree to write a letter of recommendation for a student only if you have the time, resources and inclination to do so. If you aren't sure you can commit to writing the letter, tell the student immediately so that he has time to ask another instructor to write it. If you don't think you are able to write a positive letter for the student, tell him that you don't think you're the best choice and suggest he ask another professor for help. Never write a negative or even a wishy-washy letter. A letter of recommendation is supposed to support a student's application, not hinder it in any way.
Ask the student for help in composing the letter. Sometimes students expect professors to write their letters without assistance, but this isn't fair to you. The student is asking you for a favor, so it is reasonable for you to request information from him. Ask him for all the guidelines that you need to follow, such as any specific information that should be in the letter and how to submit it. Request copies of the student's transcript and resume if you'd like a broader picture of his academic history. If he still has past papers or assignments that he has done for your class, ask him for copies so that you can refresh your memory on the quality of his work.
Address your letter to the admissions committee at the school to which the student is applying. State your name, job title and your relationship to the student. For example, if the student has taken four of your classes, write that. If you have worked with the student on an independent project or if he has worked as a teaching assistant for you, say that. Make the level of your experience with the student clear so that the admissions committee understands how much experience you have had with the student.
Give concrete examples of the student's work. State what grades he has earned in your classes and describe his work ethic. Write whether he comes to office hours frequently and routinely participates in class. Try to mention something unique about him, whether it's about his academic success in college or his personality. Give an accurate, descriptive assessment of your experience with him. Provide information that is relevant to medical school. The admissions committee will be more interested in his performance in math and science-related activities than in other subjects.
Mention other projects or activities with which the student is deeply involved, even if you have not participated in them. It's great that the student has earned three A's in your classes, for example, but if he's also the president of the Future Doctors Club on campus, tell the admissions committee about his involvement. You will need to rely on the student to provide information on his extracurricular activities to help you complete your letter.
Submit your letter on time. This is extremely important, as a late letter may disqualify the student from admission, even if all of the other components of his application were received on time.