Mix together the salt, sugar and lime zest in a shallow dish. Rub the rims of your margarita glasses with a lime half and dip into the salt mixture. Fill the glasses with ice and set aside.
Add ice to a large cocktail shaker and add the lime juice. Add the tequila, orange liqueur and agave nectar and shake vigorously until thoroughly chilled. Pour into the ice-filled margarita glasses. Top each glass off with beer.