Fill a cocktail shaker with ice. Add vodka, triple sec and juice; shake. Strain into a chilled martini glass. Garnish with a lemon curl.The Cosmopolitan provides a perfect basis for do-it-yourself mixology, and variations are numerous. Typically, variations involve changing the types and proportions of juice or using a flavored vodka, especially lemon and other citrus flavors. In particular, see the Secret Cosmopolitan and the Madras.