A thirtieth birthday party when Tequila shots were brought round reminded me of an occasion many years ago (in fact in about the year the birthday girl was born) when my parents entertained a Mexican at our house in East Dulwich.
I have no idea how he ended up staying with us but he brought with him a bottle of Tequila and gave us a lesson in how to drink it. We were all assured that his was the authentic way. There were the three stages. A pinch of salt on the bridge between the thumb and the index finger to lick, a swig (no shot glasses!) of the Tequila, and finally a sip of tomato juice from a separate glass. Somewhat extraordinarily, I was permitted to participate. Personally, my favourite bit was the salt and lemon juice.
Since then, I have never seen anyone drink Tequila in that fashion. Instead the form appears to be to start with unadulterated salt, then to down a shot of Tequila and finally to suck on a lemon slice. That was how we did it at Katy's thirtieth, anyhow. One of the deleted scenes from "The Office" involved an interesting variation on this theme in which the lemon slice was held between somebody else's teeth.