Using the PICO model, you should consider:

Patient Population - Who are your patient and his/her cohorts?
Intervention - What action are you considering?
Comparison - What are the alternatives?
Outcomes - What do you expect to accomplish?

See also The P I C O Model (Evidence-Based Medicine)

What type of question do you want to ask?

Therapy - What treatment and outcomes?
Diagnosis - Should I perform this test?
Prognosis - What is the outlook for the patient?
Harm - What is the relationship between a disease and a possible cause?

See also Types of Clinical Questions (Evidence-Based Medicine)

Sources & Further Reading

Formulating Patient Centered Questions (Evidence Based Medicine: Finding the Best Clnical Literature.  University Library, University of Illinois at Chicago)
http://www.uic.edu/depts/lib/lhsp/resources/pico.shtml

SUNY Downstate Medical Center Evidence Based Medicine Tutorial
http://library.downstate.edu/EBM2/contents.htm

User's Guides to the Medical Literature (Robert M. Hamm, Ph.D., University of Oklahoma Health Sciences Center)
http://www.fammed.ouhsc.edu/robhamm/UsersGuide/MAINPAGE.htm