Combine all ingredients in a blender with 3/4 cup of crushed ice and blend until smooth.
Pour into a chilled glass and garnish with grapefruit and basil, if desired.
Use a good quality blender that's capable of pulverizing ice into a uniform texture.
Freeze the liquid ingredients (minus the tequila or mezcal) in advance, if possible. This will help create a colder, smoother, less watered down drink. If freezing ingredients, add a 1/2 cup of crushed ice (rather than 3/4 cup) when blending.
Use crushed ice rather than cubed ice for the best texture.
Chill your glassware ahead of time to keep things extra cold.