Cut a lime and rub edges of glasses with it. Then dip the glasses in a plate that has Tajín seasoning.
Combine tequila, lime juice, triple sec, agave syrup and Tajín in cocktail shaker with ice. Shake for 20 seconds. Strain in a margarita glass and garnish with a sliced lime.
Notes
Use silver tequila when making the margarita. Gold tequila can muddy the flavor.
Fresh lime juice is the only way to go. Store-bought doesn't taste as good and is actually more expensive.
Use agave syrup instead of simple syrup. It takes takes less work and adds additional flavor.
Use crushed ice which will make the margarita colder quicker.
Adjust the spice level to your liking as Tajín seasoning can be quite spicy. Start with a small amount and add more to taste.