When I used to live in America, I went to Starbucks everyday to buy iced coffee. I would order a "venti" Iced Coffee, and "venti" is Italian for massive.
It's $2.95 for a venti iced coffee, but here's the kicker... it's only 50c for a refill. So, all you need to do is keep the same cup, and you have refills for life. Now, if you want to do this, there's a few things you should do:
Fill your empty cup with ice - Very often barrista's would become suspicious that I wasn't buying a new venti each day, to combat this keep some ice in an esky somewhere, and fill your cup to the brim. So then when the barrista says "did you buy that today?", you say "look, there's ice in it, I just drank it".
Tip - If you're getting a suspicious vibe of the barrista give them a tip, preferably cash. It doesn't gave to be much, $1 will do. And make a big song and dance about it, so they remember that you're the guy who tipped. This should buy you 3-4 unquestioned refills.
Accent - If they're onto you, put on an accent. I use British a lot and it works well: "give me my refill please... I'm better than you".
Disguise - If you're going for the accent then why not go all the way with a wig or some make up.
Good luck, you will save money doing this, exactly $2.45 per refill. The more you drink the more you save.