Have you ever seen the huge 20 ounce bottle machines, where everything is completely visible inside? When you put in your dollar and choose your flavor, the bottles move forward like in a snack vending machine, knocking the frontmost bottle down into a trap door, where it is retrieved? I don't know how common they are anymore, but if you encounter one, this is worth trying:
This works best with a friend. Simply have him/her stick his/her hand into the area where the pop falls out when it's purchased. They should be able to easily keep the trap door closed. Now buy a soda. Keep that door closed! After a few seconds, the machine will give you a credit for another soda, because it thinks the bottle got jammed somewhere. Each machine has a certain threshhold where it will not give you credits anymore, generally you can only do it twice, and then your money is gone. I remember when I first learned about this trick at my first tech support job...another guy about my age excitedly explained "You can buy three sodas for the price of one!," and went into detail about how to go about doing it. I soon realized that it was possible to buy infinite (well, not quite) sodas for the price of zero, by simply going for two drinks at a time, then hitting the coin return, letting the two pops and my dollar fall out, then starting the cycle anew. Because it was so seemingly obvious and easy to completely empty out the machine, it disappeared within weeks of everyone catching on to the trick. I doubt such machines exist anymore, and if they do, they probably don't give credits out...