I used to drink a liter or so of white grapefruit juice before dosing dextromethorphan, so it's possible that taking a therapeutic dose (30-60mg) of DXM would be an adequate substitute as they interact with the same enzymes (although I have not tested this). In my experience, however, trying to potentiate opiates with cimetidine worked somewhat, but also caused me to get the jitters and suffer an anxiety attack. Grapefruit juice works pretty well, but as Lorne??? said above you would have to drink pretty much a whole 2 liter bottle, and if you don't like grapefruit juice it's a pretty nauseating task.
A method that I have found rather effective (and delicious) is to eat a high fat meal with your opiates. I don't know the exact science behind it -- and it probably has nothing to do with CYP inhibitors -- but I used to eat one of those Banquet sausage and gravy pot pies after I took my oxy... talk about potentiation

It made me feel like I took a whole 80 when I had only taken 40.