Letters of reference (a.k.a. assessments which are written by referees) are required by most medical schools. It provides an opportunity for an admissions committee to see what other people think of you. Consequently, it may be viewed as an important aspect of your application package.
Choose the people who will submit your letter of reference in accordance with instructions from the medical schools to which you are applying. If no such instructions are given, then construct a list of possible referees. Choose from this list individuals who: (i) you can trust; (ii) are reliable; (iii) can write, at least, reasonably well; (iv) understand the importance of your application; and (v) can present with some confidence attributes you have which are consistent with those of a good physician.
Often students either want or are told to have someone as a referee who they do not know well (i.e. a professor). In such a case choose your referee prudently. If they agree to give you a recommendation, give them your resume, curriculum vitae, or any other autobiographical materials you may have. Alternatively, you may ask to arrange a mini-interview. Either way, you would have armed your referee with information which can be used in a specific and personal manner in the letter of reference.
Ideally, one or two of the referees would be professors, and one would be a medical doctor (either the second professor, a family friend or your own physician). Clearly, a powerful letter of reference would be one where the author, a doctor, strongly supports the idea of you becoming a doctor. Regarding the professor, if feasible, consider determining if one of your professors is or was a member of the admissions committee. Some professors openly volunteer such information and some medical schools publish the names of the committee members. Once again, such a referee can be very effective.
People are not paid to write you a letter of reference! Therefore, make it as easy as possible for them. Give them an ample amount of time before the deadline for submission. Also, supply them with a stamped envelope with the appropriate address inscribed. Besides being the polite thing to do, they may also be impressed by your organization. And finally, once the letter of reference has been sent, do not forget to send a "Thank-you" card to your referee.