In a pitcher, combine the lime juice, Cointreau and tequila. Cover and refrigerate until cold, at least 1 hour.
Just before serving, pour enough salt into a saucer to cover the bottom. Rub a lime half over the rim of each champagne glass and upend into the salt to crust it lightly. Pour about 3 ounces of the tequila mixture into each glass, fill the rest of the way with Champagne or sparkling wine and hand to one of your lucky guests.