My opinion, from best to worst: promethazine, hydroxyzine, diphenhydramine, doxylamine. The first two are prescription only, the second two are OTC. For something like diphenhydramine I'd recommend 50mg to start then boosters of 25-50mg as necessary. (I can take huge doses of antihistamines, but not everyone can tolerate that.)
As for whether or not it works..... you aren't kidding about contradiction here. Its never worked for me, not in any more than a hydrocodone + diphenhydramine sense (the synergy of the two rather than any actual potentiation), but others say it does, and I suppose I can't dispute what they say. If you think it'll work for you, or want to give it a shot, try 400-600mg tagamet or the correct kind of white grapefruit juice ~45 minutes before dosing the hydrocodone. Or just try the antihistamine route, because if they don't 'potentiate' they at least add sedation (which some people feel adds to the opioid feeling, and some feel it ruins it).