Quick tip to keep the amount of change in your pocket to a minimum. Suppose you only have a large bill in your pocket, and you want a soda (which of course is the only appropriate term for the substance). The soda machine will always stop accepting money after you've met or exceeded the amount of the highest price item.
The solution? Put in the change followed by your large bill. With any luck, you'll get dollar coins rather than a mess of smaller change. For example, I paied for a $1.35 soda the other day with 35 cents followed by a $5 bill, resulting in 4 dollar coins rather than 3 dollar coins, 2 quarters, a dime and a nickel.
I, of course, think the typical soda machine user interface to be fatally flawed due to the inability of it to take more money than the highest price item (although I understand that the manufacturers don't want the machines to become change machines). That debate we can have another time...for now, enjoy the workaround.