If you belong to a pre-professional group on your campus, most have programs in place to help you get in touch with someone who is amenable to being shadowed.
If you have a family physician, you could ask them if there is anyone they could introduce you or recommend you to.
Otherwise, you march up to whoever it is you want to stalk... i mean, shadow... look them in the eye, introduce yourself with a firm handshake, and say "I'm working on applying to medical school in a year or so, and I was wondering if you'd feel comfortable having me shadow you for a day or so?"
Make sure you're dressed professionally, and show them you're earnest.
I've never had this tactic fail me.
If you get along well, and they see that you've got the qualities they're looking for, these "shadow-ships" can sometimes be extended and you end up with a great mentor and letter writer.
Best of luck.
