A few thoughts:
CME - offer it for free to physicians with the catch they need to click through an ad to collect the CME. Selling the ad space is how you make money.

Specialty Specific - If you can focus the newsletter and CME on a specialty such as Radiology, Family Practice, etc, it will be much more appealing to a physician as opposed to a generic "medical" newsletter.

Click Local - If you can make the newsletter appear local it will also be more attractive to physicians. For instance, the Pacific Northwest Primary Care Montly update with free CME.