Spread a thin layer of kosher salt onto a small plate. Wet the rim of a margarita or highball glass with lime juice, and dip into the salt (rimming the glass with salt is optional - skip if you prefer and add a pinch of salt directly to your margarita instead). Fill glass halfway with crushed ice and set aside.
Stir together 1 ounce freshly squeezed lime juice, 1 ounce freshly squeezed lemon juice, 2 ounces tequila, 1 Tablespoon superfine sugar, 1-2 dashes orange bitters and a pinch of salt (make it a large pinch of salt if you are not rimming the glass with salt). Stir until the sugar and salt are dissolved.
Fill a cocktail shaker with ice and pour the margarita mixture into the shaker. Shake vigorously for 10-20 seconds. Pour into prepared glass. Garnish with a lime slice or wedge if desired.
Notes
Superfine sugar is just what it sounds like - sugar that has been finely ground. To make your own, simply put granulated sugar in your food processor or spice grinder and pulse until it is finely powdered. Do NOT use powdered sugar (aka confectioners or icing sugar) in it's place, as it contains cornstarch and this is not something you want in your cocktail.