Grapefruit juice is the only thing that will literally increase the amount of opiates in your blood.
Many people claim that things like promethazine, dxm, diphenhydramine, alcohol and benzodiazepines enhance the opiate high. In my opinion, these combinations will merely enhance the 'sedation'.
If the sedation or nod is what you're after, otc combinations are where it's at. The dopaminergic euphoria will be unchanged though, and if anything more sedation = less euphoria for me because being half asleep is, well... boring.
Eat a fatty meal right after taking the opiate.