What I always did (and helped me tremendously) was reading up on the "bread and butter" pathology that your doctor typically sees day in and day out. Maybe take the first two days in clinic to get a feel for things, but write down any terms you don't understand. Keep a list of anything cool or interesting you see. When you go home, do a little bit of research so that you can speak (roughly) knowledgeably to your doctor the following day and ask questions. The more you learn, the more questions you'll have, the more you'll have to learn (and on, and on, and on...)
Not only will this help in grasping the material, but you'll also start developing some really good self-study habits. Being able to learn and read on your own (without the prompting of another person or syllabus) will go a long way into training you for medical school. Plus, you'll be showing your doctor that you actually have an interest in the work, and you'll more than likely get a stellar LOR out of it (unless you do something to piss him/her off).
Oh yeah, one more thing: BE HUMBLE AND COURTEOUS. Do not be a d*ck to anyone you meet. Not only is it bad business, but it always gets back to the person (doctor) you are trying to impress.