Saw this on another site,
You can get free Starbucks with an empty Starbucks gift card. You could alao use one that has money loaded on it and nothing will be taken out.
When I figured that it would work, I bought holiday coffees and a bunch of food I normally wouldn't buy. I also bought some of their merchandise, but nothing really expensive in case the barista would catch on. This still works today, so for obvious reasons I won't say when to do it (that's a hint). However, it only works in certain circumstances and you wouldn't be able to do it often anyways.