If you would like salt-rimmed glasses, begin by running a lime wedge (the juicy part) around the top rim of a glass.  Fill a shallow bowl with salt, and dip the rim in the salt until it is covered with your desired amount of salt.  Set aside.

Add grapefruit juice, tequila, lime juice and Cointreau to a cocktail shaker, and shake or stir until combined.  Taste, and if you would like it to be sweeter (most people do!), stir in a half teaspoon of agave, and then repeat as needed until the mix reaches your desired level of sweetness.

Fill glass with ice.  Pour in the margarita mixture over the rocks.  Serve immediately, garnished with an extra lime or grapefruit wedge if desired.