Pour enough salt on a small plate to cover it. Rub the lime wedge on the outside rim of a 4-ounce cocktail glass. Holding the glass at a 45-degree angle, dip the outside rim in the salt, rotating as you do, so the entire rim is coated with salt. (Avoid getting any salt inside the glass.) Fill the glass with ice and place it in the freezer to chill. Add the tequila, triple sec, and lime juice to a cocktail shaker and fill the shaker halfway with ice. Shake vigorously until the outside of the shaker turns frosty. Strain the margarita into the chilled glass and garnish with a lime slice.