When I was a kid, about 7 years old, I had a sneeze that didn't want to come out when I noticed the sun glaring off a car windshield. As soon as I looked at the light, I sneezed.
Since then, I've used light my entire life(I'm 30 now) to help a struggling sneeze along. Not just the sun. I use light bulbs a lot, especially in a dark room at night. If I feel the need in that situation, I just turn a light on and look into it. I use the light shining off of snow also, by staring directly at the snow.
There's really no staring involved in it for me anymore. I used to concentrate on the light for a few seconds in order for it to work. Then, in my early twenties I noticed that when I did that there was some accompanying body language involved. My breathing would enter a certain rhythm and the muscles around my eyes and nose would flex in a certain way just as the light caused me to sneeze. Now I just flex those same facial muscles at the same moment I look into the light and it happens instantly.
In the last few years I've noticed that I can use that technique in combination with softer light, such as a television screen, a computer monitor, the screen on my cell phone, and even at certain times a cigarette lighter.
I can also cause myself to sneeze by tugging gently on an eyebrow hair or nose hair.
These techniques only make me sneeze if I want them to.
If I sneeze simply because my body needs to sneeze without any help from me, I sneeze twice. Otherwise, if I make myself sneeze, I can do it as many times as I want until my eyes become overly accustomed to the light source that I'm using. I've sneezed up to 12 in row before on purpose, to prove these things to my friends.
I've never met anyone else that can make themselves sneeze or that use these techniques to do so. When I tell people about it, they respond as if I'm joking or lying. Finding this thread is like the light source I needed to invoke a sneeze that's been waiting my whole life. Feels nice.