Another idea is to ask for
water (which is usually
free). The cup given might be
smaller, but you could go and refill it with
soda in establishments where there is no
supervision.
What amazes me about free refills is that people often still buy large or even extra large cups, yet you would have the same amount of soda (unlimited) with a small cup, and save a few dimes or quarters in the process. Of course, you might have to make an extra trip or two to the soda machine.