Fill a cocktail shaker 1/3 of the way full with ice. Add the tequila, Cointreau, lime juice, and agave. Place the top on the shaker and shake vigorously for 15 seconds. Pour a little out, taste, and if it's too tart for you, add another drop or two of agave and shake again. Fill two glasses with ice and pour over the margarita. Garnish with a lime and enjoy. Note: if you want a salt rim, run a cut lime along the rim of the glass, turn over, and dip into a plate with a thin layer of salt.