Here's what I do, step by step, when I go to fill up. Pull up to the pump, wait for the guy to walk up to my car, pop open the fuel filler door, tell the guy very clearly how much I want of what type of fuel (ex. Dame 500 de la regular), normally they repeat it to me and I make sure he's got the right amount, watch the screen as he pulls the nozzle out (of the correct fuel choice) and make sure it's reset to zero, then keep an eye on the screen while the car is being filled up and make sure it reaches the requested amount then pay cash (preferably with the exact amount). Being attentive to the whole process is key. If you're distracted talking on the phone or chatting with a passenger, chances are they'll take advantage of that. If you ask them to fill it up (lleno), then have them round it up to the nearest 50 or 100 or they probably won't have the exact change (supposedly). Just put all the odds in you favor during that 3 minute transaction and remember that in their eyes, as a gringo you have a limitless bank account so who cares if you lose a few pesos?