An important part of all research and particularly of Evidence-Based medical research is carefully formulating your research question. It's much easier to find the information you're looking for when you take the time to outline your question. One formula for mapping out your question is PICO.
The PICO model will help you to construct your "foreground" clinical questions. Your question should address each of the following areas.
Once you have identified criteria for each portion of PICO, you can use those keywords in your searches.