Ok, first the disclaimer: A] I have no idea where I learned this, although I suspect that it is one of the few useful things I took from college and B] I place no guarantee of any sort, and if it does not work at all and you end up feeling silly please don’t be angry with me, however to date it has never not worked for me (for me to cure other people, that is). Ok.
The idea is that you get the person’s attention, have them focus entirely on thier hiccups and indicate to you that they are about to hiccup. Usually the person thinks you are going to try to startle at a precise moment, but (here’s the good part) it’s the act of concentrating on the hiccups itself that makes them go away. I hope. I usually say something like “When I say to I want you to concentrate completely on when a hiccup is coming and move your hand to indicate that to me. Focus on your diaphragm muscle, you can tell a moment before it spasms. Ok? Now concentrate.” (Cheesy I know, but if it works they forget, and hopefully buy you a drink.)
So the person looks at you, poised and ready to signal, and they wait, and wait… and then the hiccups just don’t return (with any luck). I have a few happy customers so far, but I’ve never tried it twice on the same person, so I don’t know if the knowledge of what you’re actually trying to do wrecks the entire thing. It doesn’t work so well on myself, then again, I can’t get myself to concentrate on one thing for more than half a second.