Pour some kosher salt into a small dish. Rub 1 lime slice over half the rim of an Old Fashioned glass (if you prefer your Margarita on the rocks) or a coupe glass (if you prefer it up). Dip rim of glass into salt. Combine tequila, agave syrup, and juice in a cocktail shaker; fill with ice and shake well. Strain into prepared glass. Garnish with second lime slice.