I usually draw from memory, or just roughly draw out the shapes, like in gesture drawing. This one I sketched while watching the movie, so that was my reference in this case. I probably should have spaced out his eyes and nose a bit more, but I think I got the helmet right, though.
When I need to get something really photo realistic or accurate, then I make several sketches of different elements of the drawing, plus reference photos and whatnot. My best work is when I'm not thinking about it too hard.