Squeeze a grapefruit and dip the rim of a glass into the juice (I like a tumbler, though by all means use a traditional margarita glass) and then in some lightly crushed — just with your fingers — sea salt flakes.In a cocktail shaker generously stacked with ice cubes, shake together the tequila, Triple Sec (or other orange liqueur) and grapefruit juice and strain into your salt-rimmed glass, adding fresh ice cubes if you like.Spritz with lime juice to taste, for added sharpness, if wished.