Putting your hands on your forehead is a natural instinct that many people have when they are stressed. There are actually acupuncture points there that are incredibly stress relieving. You can do this for a minute or two when you are feeling stressed, or do it for a few minutes just before bed.
Somebody once told me the same thing at one of those company de-stress retreats corporations occasionally have for overworked employees. Holding your thumb, as well, can be very effective when it comes to relieving stress, I've found, for much the same reason.