I have had this happen to me on a very, very, regular basis all the way back to High School and I'm 41 now. I have always assumed it is because I have pretty low blood pressure, but never asked a doctor about it. Periodically I have tried to find something to make it stop but have never been successful and have assumed it is just something I have to live with. Until reading this thread I didn't realize it is so common. My "cure" is the "lean against the closest object until it goes away" method.