Try to get to know cardiologists. One thing I would definitely do (that I did NOT do as a student b/c I didn't think of it) is to call the secretary of the doc who is head of the cardiology dept. at your teaching hospital. Ask for an "informational interview" and bring your resume or CV with you. Ask to talk to him for 15 minutes or so about how to become a cardiologist. If you are a preclinical student, he might then remember you by the time you are a 3rd year. He will remember you were interested and proactive. At the time of your interview, if he seems like someone good to work with, you could ask if he has any opportunities for students to do research with him, or knows of any other faculty who do.
p.s. I am a cards fellow