Skip the mix and make a real margarita with this recipe.
Don't be scared to go heavy on the Cointreau.
If you don't have Cointreau, any triple sec substitute will work.
Vigorously shake all ingredients in a shaker and strain into a lowball with ice. Optionally, you can garnish with a lime wedge.
You may want to salt the rim of the glass (rub a lime wedge then dip the glass in a salt bowl), but this isn't necessary. Ask first.