For the sugar rim: In a small bowl, combine the sugar, salt and orange, lime and lemon zests. Pour onto a rimmed plate. Use the cut side of a lime half to wet the rim of 2 chilled martini glasses. Dip the rim of the glasses into the sugar mixture to coat.
For the martini: Add 1 cup of ice to a cocktail shaker. Add the vodka, limoncello, orange liqueur, lime juice and orange juice. Put on the lid and shake vigorously. Strain into the martini glasses and serve.