Run one lime slice around the rim of a margarita glass, then squeeze the excess juice into the glass. Put a Tablespoon or so of the Margarita Rimmer on a plate and press the rim of the glass into it to coat it all the way around.
In a shaker, combine the tequila, triple sec, and both cocktail mixes. Add a few ice cubes if desired and shake until blended. Pour into the salt-rimmed glass and garnish with an extra lime slice.
If you prefer a sweeter margarita, you can add agave, 1 teaspoon at a time, until the desired flavor is achieved. You can also make a simple syrup and use that instead.