1. Bring sugar and 1 cup water to a boil over medium-high heat; boil, stirring occasionally, 5 minutes or until sugar dissolves. Remove from heat; cool to room temperature (about 30 minutes). Refrigerate in an airtight container up to 1 week.
2. Stir together sugar mixture, tequila, lime juice, and orange liqueur in a pitcher. Serve over ice with lime slices.