Pour some salt onto a plate. Rub a lime wedge (or a piece of grapefruit) around the edge of a rocks glass. Roll the rim of the glass in the salt to coat the rim.
Then, fill the glass with ice.
Add the vodka (or gin) and top with grapefruit juice. Give it a gentle stir.
Garnish with a slice of grapefruit or lime.
Notes
Use fresh citrus juice if at all possible. It will give your drink a sweeter, fresher flavor than store-bought juice.Room-temperature citrus will produce juice much easier than refrigerated citrus.Use citrus, not water, to rim the glass. The sugars in the juice help the salt to stick to the rim better.Tequila can be used instead — call it a "salty chihuahua."