If you would like salt-rimmed glasses, begin by running a lime slice (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 gin, lime juice and Cointreau to a cocktail shaker, and shake or stir until combined.  Taste, and if you would like it to be sweeter, stir in a half teaspoon of agave or simple syrup at a time, 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 wedge if desired.