These threads always amaze me.
Stop calling. Stop emailing. Sit down, type out a brief 1-page resume. Type out a brief 1-page (0r less) cover letter explaining a little bit about yourself (school you go to, year you're in, that you're interested in X specialty and would like to shadow a doctor in that specialty to get a better feel for it, etc). This works best if you type up a template where you can replace the doctor's name and specialty easily. Find a handful of doctors you'd like to shadow and customize the cover letters to them. Print off copies of your resume and cover letter. Put on a nice outfit (slacks, dress shirt, nice pair of shoes) and walk into those doctor's offices. Ask the receptionist if they can pass the letter on to the doctor. Say thank you and leave.
I'm 15 for 15 using this method, split evenly between a very rural area and a very metropolitan area saturated with medical/pre-medical students vying for shadowing spots. It's a lot easier to say "no" to someone over the phone or over email than it is in person. It's also very helpful when the receptionist can comment on how professionally you handled yourself, including being appropriately dressed, and when the doctor can read a little bit of something written by you to ensure that you're not a total nut.
