Use water, lime juice or honey to wet the outer rim of your glass. Dip in coarse salt, creating a salt rim. Ensure that there is no salt on the inside of the rim.
Fill the glass with ice. Pour in vodka, then grapefruit juice. Stir to mix.
Garnish with a grapefruit slice.
Notes
For added citrus, add some grapefruit zest to the salt before creating your salt-rimmed glass.
Omit the salted rim to make a Greyhound cocktail.
Other garnish ideas: a grapefruit twist, fresh herbs, maraschino cherries.