A good margarita depends on the quality of its ingredients; always use the best tequila you can find. Margaritas can be made in a blender or served on the rocks, according to taste. Use a citrus reamer, a small, fluted cylindrical wooden tool, to extract every last drop of juice from the limes.
To salt-rim the glasses, rub a cut lime around the rim of each glass. Fill a saucer with salt, and dip each glass, upside down, into the salt. Set aside.
Fill a blender jar three-quarters full of ice cubes. Pour tequila, Cointreau, Grand Marnier, lime juice, and simple syrup over the ice. Begin blending on low, then increase the speed to high, until the ice is very finely chopped and the mixture is frothy. Taste for sweetness, adding more simple syrup if necessary. Blend a few seconds more, and pour into the salt-rimmed glasses. Serve immediately.