How would I track a face turning around? I want to track someone turning around in a scene and apply a 3D helmet to the track. How can this be done? when I tried to do it using some handy dot stickers, I did not know how to track what was on the front of my face after I turned around.

I want to take a 3d soldier helmet and mask it on my actors head.
2D Tracking is quite simple and easy to do, just take advantage of the dots and use them as a guideline in your 3d scene to make the rotation of your helmet.  

  

once you set up your tracking scene correctly and you turn your trackers into empties, you can extract transform information from the empties and apply them on your helmet object, by using a Constraint , a hook or even keyframe and rotate the helmet manually if it's a simple mesh.