Hmm... I just hold my breath and grip the back of my neck with one hand between C3-5. No idea what the underlying mechanism is (must be something to do with the phrenic nerve) but it has worked for me every time, my girlfriend, and a few of my friends that I've tried it on. It's weird because it really is instantaneous, sequence of events: hiccup, grip, gone... It's worth a shot. Try it next time you have hiccups.