Rub a lime wedge around the edges of a margarita glasses and dip immediately in the margarita salt, if desired, turning to coat. Place in the freezer for 10 minutes.
In a blender, combine the ice, lime juice, tequila, Triple Sec, Simple Syrup, and guava juice and blend until combined and ice cubes are crushed. Pour into the chilled glasses, garnish each with a lime slice, and serve immediately.
Combine sugar and water in a small saucepan and bring to a boil over medium-high heat. Cook until sugar is dissolved, about 2 minutes. Remove from the heat and cool completely.
Yield: about 1 1/2 cups simple syrup