As everyone has pointed out that to increase ones actual euphoria, potentaters and the like are not going to help out much, and so other "drugs" as in recreational drugs by themselves can be used. Many mentioned other "downers" like benzos and alcohol, but you should take it very easy with those as it can really upset the stomach and make you nauseous (alcohol in particular) or become fatal (both) due to respiratory depression.
I however would like to mention mixing in some Cocaine or Crack. And this is not at all meant as a judgment but seeing as you are posting specifically about Hydrocodones this leads me to believe you may be a little less experienced with drugs in general, Cocaine/Crack maybe hard for you acquire, or you may not at all desire to try either on their own or mixed with your Hydrocodone. But i can tell you, that being high on an opiate (usually Heroin for me) and taking a nice big crack hit is pretty much one of the most euphoric sensations I know of, only sex and orgasm come to mind as something better.