agree with others; the diaphragm is the muscle underneath the lungs that controls your breathing. Hiccups are caused by irritation of the diaphragm. knowing this; you are looking for methods of relaxing the diaphragm. that includes generally trying to work out the control for the automatic muscle; and figuring out how to calm it down.

as for trustworthy, or better-than-anecdotes - you can get surgery if it's a long-term (over several months) problem. how do you relax the diaphragm? for your human-hardware? likely different to other humans' hardware - so not much luck finding non-anecdote solutions.