Mix tequila, fresh lime juice and Grand Marnier in a cocktail shaker with ice. Shake well - until outside of shaker is frosted. Dampen the rim of an old-fashioned cocktail glass or (preferably) a margarita glass with lime juice (use a cut wedge of lime for this purpose) and dip rim in a saucer filled evenly with Kosher salt and fill with ice. Pour contents of shaker into glass. Garnish with lime wedge.