Pre-pay at the pump.

When you pre-pay with cash, the attendant makes it so that your gas can't possibly go past the money you gave them. If you gave them $30, when you're pumping your gas, the gas stops pumping at $30. This is nice, except you can't do the same thing with credit cards. It's more a pay-as-you-go with credit cards. It'd be nice if you could swipe your card, enter in the amount of gas you want, and pay less attention to trying to not cross the .00 boundary so you don't have to pull out your copper.