Use a tequila that's 100% agave. What does that mean? Dictionary.com:
aga·ve definitionPronunciation: /ə-ˈgäv-ē/
1 cap : a genus of plants (family Agavaceae) that are native to tropical America and to the southwestern United States, have spiny-margined leaves in basal rosettes and tall spikes of flowers, and include some that are cultivated for their fiber or sap or for ornamen
The bartender used the recipe borrowed from Gaston Martinez, a "brand ambassador" for Milagro Tequila. I've reprinted his recipe below for the "World's Freshest Margarita":
Always use fresh lime juice.
Use agave nectar as your sweetener.
Water is a key ingredient in a good margarita-shake well so some of the ice will melt.
Always pour your margarita over fresh ice.
2oz tequila (Gaston suggests Milagro Silver tequila)
1 oz Agave Nectar (Milagro also makes this)
1 oz Lime Juice
Served in a Rocks Glass
Garnished with a Lime Wheel