Best thing I have found is put your feet in warm to hot water an a cold compress on the back of your neck. This redirects the blood flow from your head and relieves pressure
Try drinking a little caffeine
Are you in the heat? Is your blood pressure high? Are eating too much sigar/are you eating enough? Do you spend a lot of time in front of electronics? Do you look down a lot? These are just times that I noticed I get headaches.