Letters of recommendation are one of your greatest assets in your application to health professions schools. U.S. News and World Report recommends these "7 Tips for Nailing Medical School Letters of Recommendation":
You should always confirm what types of letters you need and how many letters you need.
You should ask recommenders who can speak to your potential in your field of interest.
Get to know your professors.
Think about the size of your class. How is your professor going to be able to write you a letter if they are not familiar with you? You should make an attempt to create a relationship with your professors if you genuinely enjoy their course or share their research interests.
Nurture your relationships.
Many students end their relationships with their professors once their final grade has been posted or the research/clinical experience ends. This is a mistake especially if you are planning to ask this person for a letter of recommendation. While you may not have to talk to them every week, you should check in with people you have connected with to keep them abreast on your latest accomplishments and any changes that may have taken place in your life.
Be clear about what you need the letter for.
Communicate exactly what you need in your letter. If you are asking for a letter for a specific program or school, you should be sure to communicate that so that you are not sending a letter with the wrong information in the body. For example, if a letter states that "Sarah is great for GW Medical School," it cannot be used for her application to Georgetown. You should communicate that you would prefer that the letter says "medical school" rather than name a specific program.
Request your letters of recommendations earlier rather than later.
Keep in mind in that your recommenders are busy people. Give your recommenders at least two months notice prior to asking for a letter.
Remember to express your gratitude.
You should always remember to send a thank you card to someone who took the time to write you a letter of recommendation as well as keep them updated on the outcome of your application.