This is an awesome tip that I learned from Dr. Jennifer Keehbauch (who is the Associated Director here at Florida Hospital Family Medicine Residency). This tips was given as part of our "Learning Center" teaching series which our faculty personally do for our residents. Enjoy!
S = Systemic Signs (such as fever, meningismus, or weight loss)
N = Neurologic Signs or Symptoms (slurred speech, double vision, abnormal neuro exam)
O = Onset Sudden ("worst headache of life", thunderclap)
O = Onset < 5 years or > 50 years (don't forget about temporal arteritis)
P = Progression of Existing Headache (increased severity or frequency or change in quality)
If a patient has any of these symptoms, consider imaging to look for a secondary cause of the headache.