Fill your rocks glass or margarita glass with ice and set it aside while you prepare the drink. A cold glass keeps your cocktail at the right temperature longer.

Pour a thin layer of kosher salt onto a shallow dish or small plate. Cut your lime in half, squeeze a small amount of juice onto a separate plate, and run the cut edge of the lime around the rim of your glass, wetting only the outer edge about half an inch down.

Gently roll the wet rim of your glass through the salt, rotating it so the salt adheres evenly on all sides. Shake off any excess salt that doesn't stick.

Empty the melted ice water from your chilled glass and fill it about three-quarters full with fresh ice.

Use a jigger to measure exactly 2 ounces of tequila and pour it into your cocktail shaker. Add 1 ounce of Cointreau to the shaker as well.

Squeeze your lime by hand or with a citrus juicer to get 1 ounce of fresh lime juice, and pour it into the shaker. Add 0.5 ounce of simple syrup or agave nectar.

Fill your cocktail shaker about two-thirds full with ice, cover it tightly, and shake vigorously for about 10 seconds until the shaker frosts on the outside.

Strain the cocktail into your salt-rimmed glass filled with fresh ice, pouring slowly. Garnish with a lime wedge wedged onto the rim or dropped into the glass.