Sneeze for self-defence
1. Tilt your head slightly downward, clench your teeth, and stiffen your neck muscles.
2. Frown. Frowning will help focus your energy, and your attention at your forehead.
3. All you do now is sneeze! AT-CHE-W-W-W-U! Use, more or less the same motions with the headbutt as you do with a big sneeze.
This is the method for a standard head-butt.
More on headbutts: (1), (2), (3), (4), (5).