Slice the lime and run a piece of the lime around the edge of the cocktail glass, so that the entire edge is coated with a band of about ¼-inch thickness of lime juice.
Using a salt rimmer or a plate with salt spread on the surface, place the glass upside down onto the salt to coat the rim with salt.
To a cocktail shaker with ice, add the tequila, lime juice, and agave syrup and shake for 10 seconds.
Strain the mixture into the prepared cocktail glass.
Add a lime wheel garnish if desired.