I usually just say no quickly or maybe make a phone call, but I'm really enjoying April from Regretsy's approach:
“Hi!” She chirped.
“No,” I said, finishing my soda.
She pouted. “But, we’re saving the environment!”I stopped. “Saving it? Like how? Like recycling or something?”“Well, yes, that’s part of it!” she said, brightening.
“Oh good,” I said, and I handed her my empty bottle.
I know what I'm trying next time.