Here's my advice.
1) Private offices are easier than a hospital setting, and usually easier than clinics
2) Offices/clinics farther away from a big university are easier
3) Be willing to accept rejection and move on. Cold calling is rough, don't be surprised if it takes 10+ offices in order to find someone
4) Ask around. If you've already shadowed a doctor in a speciality, then ask if he or she knows a PCP you can shadow. Also, the normal advice of asking family and close friends about their own doctors
5) Be persistent. I ended up calling one place about 6 or 7 times. I finally got to do it, a month and a half later. It took a while to talk to the right person (usually the office manager), and she told me that the doctor already had a shadow for the next month. So I called a month later, I told her that I had talked to her a month earlier about it, and I was able to start shadowing soon thereafter. I'm convinced that my persistence convinced her that I was serious.
6) Always be courteous and polite, even when you're rejected
7) Be very flexible if your schedule allows it. If they ask you "How much were you looking to shadow?" I would recommend saying "I'm looking to get a good exposure to the field. What do you recommend?" or "How long do you normally have students shadow?" etc.
Other people have recommended dressing up in business casual, and visiting offices and clinics to show them you're a normal person and that you're dedicated. I never had to do this and I ended up getting 33 hours of PCP shadowing (family med and peds)