It's pee pH that can slow excretion of unmetabolized amps (or speed it up). Amphs excretion is strongly affected by urinary pH, and faster the more acidic it is.
It's not easy to change your pee pH, but mega doses of vitamin C would do it. That much has data out there, but I bet phosphoric acid, as found in the dark-colored sodas, has an effect (how much diet coke? no idea).
Other things can acidify your pee, but you'd risk hurting yourself more than an amph overdose.
And then, aim for citric acid salts and blood pressure meds (thiuridizide class) to keep your pee pH more neurtral for retention.
Don't eat a box of baking soda, even if it could work. You'd be pretty funny foaming from your mouth and nose and choking to death but then you'd die.
EDIT: missed you tubbs, but there's two things: absorption of oral amphs by your GI tract is dependent on pH, and that's separate from excretion of unmetabolized amphs (pee pH).
So acidic foods mean less absorption. Acidic pee means faster excretion.
All the texts out there blame it on the amine, but that makes no sense. Or at least, it's gotta be way more complicated than that. You [wikipedia] want me to believe you can't absorb a water soluble molecule out of your intestines? The makers of Vyvanse believe that.