Introduce yourself and your relationship to the student in your letter. The introduction is important because it establishes your authority to gauge the applicant's qualifications for medical school. In addition to your title, note any special positions you hold, recognitions you have received and experience in teaching. For example, if you have spent 10 years teaching thousands of pre-medical students, your letter will give you greater authority than a new professor teaching his first class. Lastly, discuss your relationship with the applicant, if she were a student or mentee of yours. If possible, be detailed to avoid appearing as this was just "another one" of your students.
Evaluate the candidate's readiness for medical school in your letter. If you are truly an advocate for this student, be as persuasive and definitive in your letter as possible. For example, say, "I have no doubt that Kirsten is not only ready for, but will also thrive in medical school." Follow this with one or two examples of why you believe this to be true. Mention times your student has produced phenomenal research or describe her willingness to ask questions in class.
Having a strong work ethic is an important characteristic to have as a medical school applicant. However, most students who apply to medical school will have a strong work ethic, so you will need to discuss the applicant's work ethic as being unlike most of her peers. To ensure she stands out, draw a comparison between the applicant and her peers. You might say, for example, "While most students work hard in my class, Kirsten has demonstrated a work ethic unlike any of her classmates. She comes to office hours almost every week and goes above and beyond in all of her assignments."
The purpose of your letter of recommendation is to provide information not found anywhere else in the application. As such, choose a personal trait or characteristic of the applicant and provide an anecdote to illustrate this trait. For example, if the applicant is very diligent, provide a story that illustrates this. Or, you may also discuss the applicant's passion for medicine, as evident through your interactions with her. When discussing these details, though, be specific and clear to paint a clear and compelling picture of the applicant.