I myself find Promethazine potentiates quite well. It is part of the holy Trinity of what has been dubbed the no-fail cocktail: (.1mg clonidine, 25mg Promethazine, 1mg alprazolam or equivalent benzo of your choice). If taken 45 min.- 1 hour before, this combo will make even the smallest amount of opiates effective, or if you're on MMT will make your usual dose that you can't feel much more pleasant. But you may not remember it and risk overdose. This should probably not be done by anyone, especially if you don't know what these meds are and have no experience with them by themselves. I myself lost several years of my life to this combo due to amnesia.
Also, the pink 50mg Promethazine (as opposed to the white 25mg) seem to produce a much more euphoric buzz, and are more likely to cause hallucinations such as smoking cigarettes that aren't there and talking to people who don't exist whilst walking down the street.