There are many cover letter samples for medical assistants online. One example is at http://www.jobbankusa.com/resumes/cover_letters_free_samples/examples_templates_formats/medical_assistant.html. The sample is great because it is too the point. You don't want to have a cover letter that is too lengthy. You can find more tips at http://www.medicalassistant.net/sample_resume.htm.
