A head harness is the best bet. Padded leather, around $30; you put it on your head (Has a strap like a headband and one over the top), has a chain hanging down that you put free weights on and do your exercises. If you buy one I'll tell you about exercises.
Some gyms have a machine (cable system I believe, with plates) that you push a padded bar with your head. Decent.
Or drills: Some are leaning against a wall, forward and backward. Or Bridging and doing the opposite of bridging (on your face). Advanced work: When bridging, etc, rotate your body 180degrees, from bridge to face plant, and on around. Don't forget the sides, turn your head.
Found pics for ya:
www.rossboxing.com/thegym/thegym15.htmPrepare yourself for headaches at first. Sore muscle, right next to the brain, that you use alot
And stares. People wonder what you've been up to.