At Safeway I used to order one of their breakfast burritos before class and would wait for them to print out the sticker before saying, "I'm sorry, could you please add guac?" Instead of printing a new sticker with a new price, they'd just use the one they had and I wouldn't pay the extra 75 cents for guacamole. It worked every time.
I would check the times at the local multiplex and figure out which two movies I wanted to see. I would buy a medium sprite, toss in some vodka and watch the first movie. Then at some point I would return to the lobby and say, "can I get a refill?"
They would say, "I"m sorry, we don't offer free refills anymore."
"Oh really? Oh well (sad face)."
"Actually sir, we will do it for you this time."
"Gosh, thanks!"
And then I would walk to the next movie and dump in some more vodka. I did that for two years and never saw the same cashiers.