The University of Washington's Pyramid of Evidence-Based Practice Tools presents a framework for searching for evidence:
Begin your search using resources from the top of the pyramid to find high level evidence with the least expenditure of time. Not all subjects will be covered by these resources. Move down the pyramid as needed to find information on your topic.
Use these resources for overviews of diseases and conditions. Bear in mind that some of these may not incorporate the latest research.
Merck Manual of Diagnosis and Therapy
Enhanced, online version of a standard medical text.
Provides overviews of over 9500 topics in 21 areas of practice.
R. Brian Haynes and colleagues developed the 6S Model: