the key is to progressively get louder and more frowny. if you can start off by smiling, they will be lulled into thinking you are a passive pussy; a good starting point.
swearing and threatening sadly bring discussions to an abrupt end these days (oh for the good old days...)
long words and a diversified arguement also make them fear that the situation could go on for some time... keep something in reserve for when you hit a brick wall; if the arguement reaches an impasse, then you lose.
and a really compelling decider is to go for it when there is a healthy queue formed behind you (giving them an audience to face) especially when people have just arrived, and / or when their shift is about to end.
believe, you pay more than you should for everything... and i mean EVERYTHING! shit, in the past i have bartered the price of a pack of rizla!
You really have thought this through. You could write a manual on this. This is why I no longer work it retail! haha. Good work though.