There might be a prize
The Arizona Cardinals, for instance, enter designated drivers into a contest that holds out the possibility of such grand prize swag as a trip to the Pro Bowl. Which is in Hawaii.
In Ventura, Thousand Oaks, Camarillo and Simi Valley, CA, a gang of youths named "The Prize Posse" roams the streets looking for designated drivers to reward with prizes worth $15-$20, and entry into raffles for pricier loot, like $100 cash, or Sublime tickets. At the annual opening of Maryland's The Tiki Bar, designated drivers have soberly exited the festivities with everything from bluetooth speakers to a 7-day cruise. (The bar is on Solomons Island, so it's unclear how much driving is involved, but still it's the right sentiment.)
There’s an excellent double entendre opportunity
At some point, one of your friends might say they’re okay to drive. You’ll tell them no. They’ll say, “No, no, I’m cool.” You’ll nod your head, smile, and say “You are not cool.” For a second they won’t be sure if you meant cool to drive or cool in general. That is a very pleasurable second.