In gadget-loving Estonia the answer is easy: hook the machines up to the mobile phone network.
Instead of inserting coins in exchange for a soft drink, the customer dials a mobile phone number. A frosty bottle appears as usual and the price of the drink is added to the customer's phone bill.
The first such machine has already been installed and may be followed by 25 to 75 more if the idea catches on.
The project is the result of cooperation between Coca-Cola Eesti and Estonia's largest mobile phone provider, Eesti Mobiiltelefon, and may be the start of a much larger trend.
In fact, EMT hopes to expand the possibilities for using your mobile phone as a payment system to other spheres, making it almost as widespread as the use of credit cards or checks in other parts of the world.
According to Raul Vahisalu, who heads the Coke machine project at EMT, it would be feasible to set up similar systems of payment for other services, such as parking. Payment by mobile phone could also be a way of aggregating payments, allowing customers to pay a single bill for many services.
"We are trying to educate people to think of a mobile not only as a phone but also as a credit card or wallet," he said.
EMT is currently considering similar projects with other, undisclosed companies.
In Finland, which leads the world in mobile phones per capita, not only soft drink machines but also juke boxes and car washes have been hooked into the mobile phone network.
Even there, however, the new generation of soft drink machines has been slow to catch on. Alain Vignard of Coca-Cola Eesti explains that this is mostly because prices are higher than through traditional payment methods. In Estonia, he says, "the price is right," and a Coke, Fanta or Bon Aqua from the new machines costs 10 kroons ($0.68), about the same as in a store.
Vignard also added that up to 75 machines will be installed throughout Estonia, in locations such as schools and transport terminals.
Soft drink machines never really caught on in Estonia. According to Vahisalu, Coca-Cola Eesti brought in the machines a couple of years ago, but soon found out that in a market where even one- and five-kroon coins are rare and bills are often soft and of low-quality, vending machines require more patience than most customers have. Most of the machines were pulled from the market.
The second time around, Vahisalu believes, the machines' high-tech allure, coupled with a bit of nostalgia, will save them.
"In Estonia in the Soviet era there were a lot of machines around," he said. "So there is like a collective memory of them. Estonians know what vending machines are."
The new-fangled machines are also attractive to businesses who might install them. Because the machines are hooked into the mobile phone network, they can keep the central distribution network constantly up-to-date on their inventory. If something goes wrong, they can summon security. Also, as there is no money in the machines themselves, they are a less attractive target for thieves.
The prototype soft drink machine is located in EMT's customer service headquarters in Lasnamae and, for now at least, also takes old-fashioned money.