The simplest idea is sometimes the best:
Stop using paper cups. Offer thermoses and mugs with the company logo on them and tell people to bring their own mug. People are too addicted to coffee on the go to stop buying it just because they have to bring their own mug.
In order to make sure that people are being charged for the correct drink size regardless of which mug they bring, the machines should be set to dispense the amount of liquid appropriate for each size or the drinks could be made in a starbucks mug and then poured into the customer's mug.
Another idea pertaining to size is that the company only serve drinks in the mugs with their logos, but I know people who go to different coffee shops throughout the day depending on what they're closest to and carrying around a lot of mugs just to get coffee would be a pain. Maybe the companies could come up with some sort of standardized sizing so that their mugs could be used at different places.