Between 2012-14, I lived in Los Angeles. I went to Starbucks everyday to buy iced coffee. I would order a "venti", and "venti" is Italian for massive.
It's $2.95 for a venti iced coffee, but 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 conspicuously - If you're getting a suspicious vibe of the barista give them a tip, preferably cash. It doesn't have to be much, $1 will do, even better if it's coins. Coins allow you to make a big song and dance about it, so they remember that you're the guy who tipped. This should buy you 3- unquestioned refills.
Accent - If they're onto you, put on an accent. I used British a lot and it works well: A British accent is easy, just say this: "give me my refill please". Whilst thinking this: "I'm better than you".
Disguise look and movement - It important to keep the barista guessing. I would alternate wearing my hair down or up, wearing reading glasses, and sometimes putting on a limp,
Good luck, you will save money doing this, exactly $2.45 per refill. The more you drink the more you save.
Check out the photo below of my inventory of Starbucks cups. It's important to keep at least 5 in rotation, this keeps the baristas guessing.