Run a lime wedge around the rim of a rocks glass (or margarita glass), and then dip the rim into margarita salt. Carefully fill the glass with ice so you don't knock off much salt. Set aside.
Fill a cocktail shaker with ice and pour in 1 1/2 ounces silver tequila, 1/2 ounce lime juice, 1/2 ounce triple sec, and 1 1/2 ounces sweet and sour mix. Cover and shake vigorously until the shaker feels very cold, about 30 seconds.
Uncover and taste. Adjust the sweetness with agave syrup to your liking, if needed.
Strain into the prepared glass and garnish with a 1 lime slice. Serve immediately.
Notes
For a richer orange flavor, swap triple sec for Grand Marnier.
For a frozen margarita, blend the ingredients with 1 cup of ice per serving.
For a bigger batch, switch ounces for cups for the ingredients, stir in a large pitcher, and chill before serving over fresh ice.