I've heard benzos can be potentiated by inhibiting the enzymatic activity of CYP3A4, causing slower breakdown of the substance in question, and leading somehow (exactly how, I'm not sure) to higher plasma concentration. I've also heard that taking a CYP inhibitor increases oral bioavailability of drugs metabolized by that particular enzyme (which is a wide variety of drugs, if I understand correctly). The easiest CYP inhibitor to get ahold of is grapefruit juice. There are also a number of pharmaceutical ones. Also, it is my understanding that a large number of drugs taken via oral administration are metabolized by CYP3A4, meaning this is potentially useful for more than just Xanax.
Other than that, just switch to a benzodiazepine with a longer half life like Clonazepam, Diazepam, or Lorazepam. You can use an equivalency chart to figure out the half-life of whatever benzo you happen upon.